- Email: [email protected]
Development of novel catalytic additives for the in situ reduction of NOx from fluid catalytic cracking units
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
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D E V E L O P M E N T OF NOVEL CATALYTIC ADDITIVES FOR THE I N S I T U REDUCTION OF NOx F R O M FLUID CATALYTIC C R A C K I N G UNITS Barth, J., Jentys, A. and Lercher, J.A. Technische Universit~itMtinchen, lnstitut ~r Technische Chemie, Lehrstuhl II 85747 Garching bei Mtinchen, Germany. E-mail: j [email protected]
ABSTRACT The chemistry during the FCC regeneration process was investigated providing a deep insight into the chemistry leading to the formation of NOx in the regenerator. From coke loaded spent FCC catalysts NH3 and HCN were determined as reduced intermediates for the formation of NOx. The improved understanding of such reaction intermediates formed during the FCC regeneration process was used to develop catalytic nanocomposite additives (derivatives of MCM-36) that promote the reduction of NO to N2. A novel approach to the synthesis of such catalysts was chosen, using layers (thickness: 2.5 nm) of zeolite MCM-22 spaced apart by clusters of MgO-A1203. The nanocomposite catalysts were tested in catalytic reactions under the conditions of the FCC process, demonstrating a significant reduction of NOx emissions.
INTRODUCTION Fluid catalytic cracking (FCC) is a key process in modern refineries. [1] Worldwide approximately 300 FCC units are operated, converting vacuum gas oil and high boiling residues into lighter fuel products and petrochemical feedstock. Due to its central function in modern integrated refineries, a range of technological improvements has been implemented, to increase the economical benefits from FCC units. In addition to investments concerning the process design, new catalysts and additives have been developed to fulfill the economic demands of the market. However, refiners are bound to invest also in eco-efficient technologies for the production of fuels and petrochemicals with significantly reduced emissions of environmental pollutants. [1] This is imposed by various stringent national and international regulations addressing emissions from a range of refinery processes and especially FCC regenerators, such as NO, SOx, CO and CO2 emissions from regenerator flue gases, t21 Approximately 2000 t/yr NOx are released from a typical refinery. The FCC units contribute to approximately 50 % of that. The concentrations of the NOx emissions from regenerator flue gases vary in the range of 50-500 ppm depending on the nature of the feed, the operating conditions of the FCC unit and the amount of CO promoter added. In the fluid catalytic cracking process, nitrogen containing species in the feedstock are cracked in the riser reactor to lighter molecules, while a fraction is deposited in the coke on the spent catalyst, During oxidative regeneration of the catalyst, more than 90% of the coke bound nitrogen is converted to molecular nitrogen (N2), while the rest is released in the form of NO. Sources of nitrogen leading to NOx formation are mainly the FCC feedstock ("fuel NOx"), while only minor amounts (< 10 ppm) are formed by N2 oxidation ("thermal NOx") and the reaction between radicals ("prompt NOx") in the regenerator. Over the last 20 years additives (Pt based CO combustion promoters) have been used to control CO emissions. However, these additives have significantly increased NO emissions. It has been speculated that such combustion promoters can oxidize reduced nitrogen containing intermediates such as HCN and NH3 to NOx. The reduction of NOx emissions from FCC units can be achieved by (i) using conventional end-of-the pipe technologies, such as the SCR (selective catalytic reduction) DeNOx process using NH3 as reducing agent, or (ii) by using additives to the FCC catalyst. These additives should catalyze the in situ reduction of NOx under the typical reaction conditions of the FCC unit. For the development of such additives it is essential to understand the reaction mechanisms, which lead to the formation of N2, NOx and reduced nitrogen containing intermediates such as HCN and NH3 under the conditions of the FCC process. In this paper we will discuss possible pathways of nitrogen containing species in the FCC unit. Especially, the nature of reduced nitrogen intermediates for the formation of NOx and factors controlling the formation of NOx from these intermediates were addressed. Novel nanocomposite materials based on
2442
MCM-22 (derivatives of MCM-36 with mixed oxide-pillars) were developed as NOx reduction additives enabling the reduction of NO with CO and reduced intermediates such as NH3 and HCN. The catalytic performance of MgO-A1203-MCM-36 was investigated under reaction conditions similar to those in the regenerator of the FCC unit. The catalysts were investigated under reaction conditions approximating the oxygen depleted and the oxygen rich zone of an FCC regenerator operating under full burn conditions. Oxygen deficient reaction conditions are encountered in the oxygen depleted zones of the dense and the diluted phase of the FCC fluidized bed. The incomplete combustion of coke leads to the formation of a reductive atmosphere in parts of the dense phase of the fluidized bed mainly due to the formation of CO and some hydrocarbons not desorbed in the stripper. Simultaneously, in the bottom region of the regenerator near the air inlet an oxygen-rich zone is encountered. The MCM-36 type materials were tested as catalysts for the reduction of NO with CO as well as in regeneration experiments mixed with spent FCC catalysts in a fluidized bed reactor. The reaction mechanism that leads to the reduction of NO by CO to N2 was studied by IR spectroscopy.
EXPERIMENTAL The catalysts were prepared as described elsewhere. TM Catalytic experiments were performed in a fixed bed and fluidized bed reactor simulating the conditions of the FCC regeneration unit. [4] A detailed description of the physicochemical characterization of coked FCC catalysts and reaction intermediates formed during the regeneration can be found in reference [51 and [6]. R E S U L T S AND D I S C U S S I O N
Pathways of nitrogen in the FCC unit As we have shown earlier, polyaromatic pyrrole derivatives (alkylcarbazoles, alkylbenzocarbazoles, alkylindoles) are the main source for nitrogen in the feedstock of FCC units, ts] These molecules dominate over 6-ring nitrogen species (pyridine derivatives) like alkylquinolines and alkyltetrahydroquinolines. After cracking 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 LD-/MALDI-TOF-MS spectroscopy as main nitrogen containing components of the coke. The majority of these species possess relatively high molecular masses (m/e = 350-850) and are probably trapped in the meso/macropores of the FCC catalyst microspheres. We have shown by TPO and LD/MALDI-TOF-MS experiments that during oxidative regeneration these large, pre-graphitic type compounds are converted into smaller carbazole and quinoline type molecules. 161With increasing oxidation temperature smaller nitrogen containing coke species accumulate on the catalyst surface. They are oxidized only after most of the non-heteroatomic hydrocarbons have been burned off. Such pyrrole and pyridine derivatives are probably trapped in that stage of regeneration on the strong Bronsted acid sites of the cracking catalyst. Such coke species are the actual precursors for the formation of N2 and NOx in the FCC regeneration unit (Fig. 1).
N 2 (g0 % of co ke IV)
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735 ~"0 --~EGENERATO 720"C
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Figure 1. Pathways of nitrogen containing compounds in a typical FCC unit.
2443
V a c u u m T P D m e a s u r e m e n t s of F C C catalysts c o k e d wi t h m o d e l s u b s t a n c e s b e a r i n g n i t r o g e n f u n c t i o n a l grou p s (aniline, pyrrole and pyridine) Temperature programmed desorption experiments in vacuum have been performed with FCC catalysts that had been coked with aniline, pyrrole or pyridine. The desorption of NH3 and HCN from the samples was followed by mass spectroscopy in order to compare the effect of different nitrogen containing precursor, species in the coke on the formation of reduced intermediates which are possible sources for the formation of NOx. Figure 2 shows the desorption of HCN (m/e = 27) and NH3 (NH +, m/e =15) from the samples. The maximum of HCN emissions follows the order of increasing basicity from pyrrole (maximum at 777~ to aniline (791~ and pyridine (813~ Pyridine derivatives in the coke are removed last from the acid sites of the FCC catalyst and form HCN. The comparison of the formation of NH3 from the coked catalysts shows a broad maximum a t - 7 1 6 ~ for the FCC catalyst coked with aniline. For the pyrrole and pyridine coked samples lower amounts of NH3 have been detected. Again, for the sample deactivated with pyridine the formation of reduced nitrogen species occurred at higher temperatures than for the less basic pyrrole. It can be speculated that in the polyaromatic coke formed by aniline cracking a higher fraction of amino (-NH2) groups is present which form ammonia upon pyrolysis of the carbonaceous deposits. The comparison of the desorption curves in Figure 2 indicates that the emission of HCN occurs at higher temperatures than that of NH3 (pyridine: 813 vs. 7 5 0 ~ 2 . 5 0 E -07
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Figure 3. Temperature programmed desorption of species from a coked FCC catalyst from a commercial FCC unit (left: p = 10.3 mbar, right: p = 1 bar, 100 ml He/min, 0 % 02, detailed view). V a c u u m T P D m e a s u r e m e n t s o f c o k e d F C C catalysts Figure 3 (left) shows the desorption of species from a coked FCC catalysts under vacuum (p = 10 -3 mbar) in a temperature range of 50-700~ The maximum of H20 desorption was observed at temperatures between 100-200~ H2 was formed during pyrolysis at temperatures above 450~ (not shown), with a maximum at 870~ and HCN desorption reached a maximum at 840~ For the identification of NH3 emitted
2444 from the catalyst the signal m/e = 15 was followed. The maximum at 620~ cannot be attributed unambiguously to NI-I+ as at the same temperature a maximum for the signals m/e = 13 and 14 can be observed hinting at methyl fragments (CH3 +) desorbing from the catalyst. However, at temperatures of -800~ the desorption curve of m/e = 15 shows a shoulder which can be attributed to NH +. This shoulder is more intense (maximum: 820~ if the experiment is performed in flowing He (100 ml/min) (cf. Fig. 3, right).
Temperature programmed oxidation experiments with coked FCC catalysts: the effect of Pt-based combustion promoters TPO experiments were carried out with samples of a coked FCC catalyst (232 ppm nitrogen; flow 100 ml He/min; 5 % 02). As it can be seen in Figure 4 carbon and nitrogen containing species are burnt sequentially. The maximum of NO formation (653 ~ occurred at significant higher temperatures than that of CO2 (570~ and CO (parallel to CO2; not shown). Figure 5 shows the formation of NO, NO2 and HCN from oxidation of carbonaceous deposits in the presence of a CO combustion promoter. In the absence of such an additive, the maximum of NO emission was observed at higher temperatures than that of HCN (653 vs. 622~ Note that NO2 is formed at lower temperatures (570~ occurring together with the maximum of CO2 formation, while NH3 was not detected in the presence of oxygen in these experiments. After addition of l wt.% commercial CO combustion promoter (Pt based additive) significant higher amounts of NO and NO2 were formed and the maximum of desorption was shifted to lower temperatures [611 vs. 653~ (NO), 558 vs. 570~ (NO2)]. The addition of the CO combustion promoter leads to an almost complete oxidation of HCN and a simultaneous increase of NO and NO2 (not shown) concentrations, of which the increase for NO2 is less pronounced. Higher oxygen concentrations in our TPO experiments, as well as higher excess oxygen in the flue gas stream of the FCCU regenerator are correlated with higher NOx emissions. It was demonstrated earlier by Zhao et al. that NOx is not formed from the oxidation of molecular N2 from the air ("thermal NOx"), but results from the oxidation of reaction intermediates such as HCN and NH3 from coke pyrolysis. [7] The present results indicate that in a typical full combustion regenerator most of reduced nitrogen species (HCN and NH3) are oxidized to NO as long as there is an excess of oxygen. Under partial combustion conditions a significant amount of NH3 and HCN should be present in the FCC regenerator, which can react then subsequently to N2 or NO. CO combustion promoters favor the formation of NO by (i) oxidizing intermediates such as HCN and NH3 to NO and by (ii) reducing the concentration of CO which can act as reductant for NO. 8.00E-10 ~ 5 6.00E-10 ~.)
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Materials The above results clearly demonstrate that an in situ reduction of NOx emissions from FCC units can be only achieved by the reaction of NO with (i) CO or (ii) NH3 / HCN formed during the pyrolysis of nitrogen containing coke species. The use of nanocomposite materials based on zeolite MCM-22 with mixed alkaline earth oxide pillars between the zeolite layers is a novel approach to enable both reactions (over basic oxide clusters in the interlayer galleries and Bronsted acid sites in the zeolitic sheets). In contrast to noble metal containing catalysts such additives should not increase NOx emissions by the oxidation of reduced intermediates such as NH3 and HCN.
2445 Table 1 summarizes the textural properties and the acid site concentrations (determined from temperature programmed desorption of ammonia I81 of MgO-A1203-MCM-36 investigated in this study. A detailed physicochemical characterization and investigation of the acid-base character of such nanocomposite materials can be found in reference t31. Table 1. Properties of MgO-A1203-MCM-36 investigated in this study. Sample
Si [wt.%]
Al [wt.%]
Na [wt.%]
Mg [wt.%]
MgO-A1203-MCM-36
27.92
21.28
< 0.10
0.64
BET s.a. [m2/g] 348
c (acid site) (mmol/g) 0.91
TEM micrographs demonstrate clearly that the materials have a layered structure. The pillared zeolite sheets form aggregates of appr. 100-150 nm length and -100 nm width. The layer thickness can be estimated to be appr. 2.5 nm which is consistent with the values reported by Roth et al. [9] The presence of pillars keeping the layers apart cannot be observed directly probably due to insufficient contrast. These less dense areas between the sheets are attributed to interlayer distances being equal to ca. 0.1 nm (Fig. 5 right). The packing of the MCM-22 sheets seems to be less regular on the borders and the outer termination of the aggregates (Fig. 5 left).
Figure 5. Transmission electron micrographs of MgO-A1203-MCM-36. Kinetic m e a s u r e m e n t s MgO-A1203-MCM-36 was tested as catalysts for the reduction of NO with CO under reaction conditions simulating the FCC regeneration process. CO is the dominating reducing agent in the regenerator, due to the incomplete combustion of coke. Figure 6 shows an example for the reduction of NO with CO over MgO-AlzO3-MCM-36 under conditions similar to the oxygen depleted (Fig. 6a) and rich (Fig. 6b) phase of the FCC regenerator operated under full burn conditions. N2, NO2 and N20 were formed as reaction products, while CO2 was generated by the oxidation of CO. In the relevant temperature region of 600-750~ high yields (50-84 %) of N2 were obtained under reaction conditions similar to the oxygen depleted dense and diluted phase of the fluidized bed of the FCC regenerator (0.5 % 02). In the presence of higher oxygen concentrations (2.4 % 02), which are encountered in the lower parts of the dense phase, significantly lower yields (10-40 %) of N2 were measured. In the first case considerable amounts of N20 were formed with a maximum yield of 50 % at 575~ whereas under the reaction conditions with high oxygen concentration only 17 % N20 was formed with its maximum shifted to higher temperatures (625~ In contrast to this observation, under oxygen rich conditions (2.4 % 02) higher yields of NO2 were obtained. For both cases the conversion of CO and yield of CO2 increased with rising temperature and reached a constant maximum degree of conversion at 575-600~
2446
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Figure 6. NO reduction with CO over MgO-A1203-MCM-36" a) 700 ppm NO, 1.4 % CO, 0.5 % 02; b) 1000 ppm NO, 4.4 % CO, 2.4 % 02. MCM-36 type additives with mixed alkaline oxide aluminium pillars (MgO-AI203; BaO-A1203) show a reduction of NOx emissions (-~20 %) even in experiments simulating the regeneration of industrial coked FCC catalysts in a fluidized bed reactor (a detailed discussion can be found in reference t61). In contrast to Pt based additives, the MCM-36 derivative does not oxidize reduced nitrogen containing intermediates (NH3, HCN), which are generated during the pyrolysis of nitrogen coke species, but lowers the concentration of NOx released in the flue gases. It is speculated that during the regeneration of spent FCC catalysts the MCM36 type additives catalyze, in addition to the NO + CO reaction, the SCR reaction of NO with NH3 to N2 on Bronsted acid sites in the zeolite layers or on the pillars in the interlayer galleries. HCN, which is a key intermediate in the nitrogen chemistry of the FCC unit might be hydrolyzed to NH3 over the basic oxide pillars in the interlayer galleries. The additives can be used in combination with Pt based CO promoters to simultaneously control the level of NO and CO in the regeneration unit.
IR spectroscopy: adsorption of NO, 02 and CO on MgO-A12Os-MCM-36 For the investigation of intermediate species formed on the surface during the reduction of NO with CO in the presence of oxygen co-adsorption of NO and 02 followed by adsorption of CO was carried out. The formation of nitrite and nitrate species (bridged bidentate nitrate, monodentate nitrate on AI, linear nitrite on AI) was observed. Carbonates (from adsorption of air CO2) on the basic material were removed upon NO adsorption. After adsorption of 1000 ppm NO in the presence of 3 % 02 predominantly nitrate species were formed on the nanocomposite materials. Bands at 1575 cm -~ characteristic of monodentate nitrates on A1 and 1552 cm -1 (bidentate nitrate on AI close to Mg) were observed, whereas nitrites (1459, 1463 cm-l: linear nitrite on A1) were only detected in minor concentrations under oxidative conditions. After adsorption of NO and 02 the sample was equilibrated with 5 % CO in flowing He. Figure 7 shows differences in the IR spectra measured during heating of the catalyst from 200 to 500~ in the presence of CO. Bands indicative of nitrates and nitrites decreased in intensity (negative bands) while bands at 1378, 1384 1588, 1610 and 1637 cm -~ first increased in intensity, went through a maximum a t - 3 5 0 ~ and decreased again with increase in temperature. Partially overlapped bands were measured above 250~ at -2280-2240 and 2227 cm -I which with rising temperature increased continuously in intensity. The first set of IR bands can be attributed to bicarbonate (1445, 1646 cm -1, shoulder at 3610 cm-I), uni- (1410, 1540-1580 cm -1) and bidentate (1378 cm -I) carbonate species adsorbed on basic sites of the MgO-A1203-MCM-36 material. Bands at 2930 (not shown) 1610 and 1384 cm -1 indicated the presence of formate species on the MgO-A1203 clusters. The broad bands at 2280-2240 cm -~ are indicative of the degenerate stretching vibration of surface isocyanate species adsorbed on the MgO-AI203 pillars (AI-NCO: 2260, Mg-NCO: 2240 cm-1). The narrow peak centered at 2227 cm-1 is tentatively ascribed to the N-N stretching vibration of N20 formed during the reaction.
2447 I"--O(30 O3 CD C0 t,.D ~ I,~
C3 CO 03
.005
U C r
o
(oO ..Q
13 200
-.005
300~ 400 .~. 500 ~ 1700 1600 1500 1400 1300 W a v e n u m b e r [crrrl]
Figure 7.Differences of IR spectra measured during reaction of 5 % CO on MgO-A1203-MCM-36 after exposure of the catalyst to 1000 ppm NO and 5 % 02. The results demonstrate that MgO-A1203-MCM-36 type materials containing mixed alkaline earth aluminium oxide pillars show high catalytic activity in the reduction of NO with CO, in the presence of oxygen, to N2 and CO2. Generally, the conversion of NO and yields of N2 and N20 were significantly higher under oxygen deficient reaction conditions, which are encountered in the oxygen depleted zones of the dense and the diluted phase of the FCC fluidized bed. The presence of higher oxygen concentrations inhibits the reduction of NO by CO as the CO/O2 reaction is favoured in expense of the CO/NO reaction. Under reaction conditions similar to the oxygen-rich zone, which is encountered in the bottom region of the regenerator near the air grid (500-625~ oxygen excess), NO is mainly converted to NO2 over the MCM-36 derivative. As a direct oxidation of NO to NO2 is not favoured thermodynamically at such high temperatures, high yields of NO2 can only be attributed to the decomposition of surface nitrite and nitrate species formed on the basic oxide clusters (MgO-AI203) in the interlayer galleries. Note that the temperature programmed desorption of NO2 from MgO-A1203-MCM-36 has shown that such ionic NOx species still decompose in the hightemperature region above 500~ Various types of surface nitrite and nitrate species (mono-, bidentate, bridged nitrates) have been observed by IR spectroscopy on the basic MCM-36 type materials. The interaction of NO with the pillared zeolites led to the removal of carbonate species present on the basic oxide clusters and the formation of different ionic NOx surface species, which are probably located on the pillars between the zeolite layers. Interestingly, nitrates were already formed in the absence of oxygen, suggesting an oxidation of NO by reactive oxygen species present in the mixed oxide clusters in the interlayer galleries. In the presence of 3 % oxygen mainly nitrate species were observed on the surface with bidentate nitrates on A1 close to Mg (1552 cm -1) being the dominant species. Our IR measurements showed that with increasing temperature the presence of CO (after exposure of the catalyst to NO and 02) led to a removal of nitrite and nitrate species from the catalyst surface. Simultaneously, formate and various (bi-)carbonate species were observed, which can only result from an oxidation of CO during the reaction with the surface NOx species. The concentration of carbonate and formate species (adsorbed on basic sites) increased on the catalyst surface u n t i l - 350 ~ before it started to decrease again, which can be explained by a thermal desorption of the weakly bound species. For the understanding of the reaction mechanism of NO reduction with CO over MgO-A1203-MCM-36 it is important to recognize that at temperatures above 300~ a distinct formation of isocyanate species and N20 was observed. The wavenumbers of the partially overlapped IR bands (2260, 2240 cm 1) suggest that the -NCO intermediates are most probably adsorbed on the MgO-A1203 pillars (A1NCO; Mg-NCO). Simultaneously with the observation of isocyanate species the formation of N20 was indicated by the intense band of the N20 stretching vibration at 2227 cm -1. Isocyanates such as A1-NCO and Mg-NCO species may be formed by the reaction of nitrite and nitrate species with CO. The formation of the N-N bond in the reduction of NO with CO leading to N2 and N20, is tentatively ascribed to the reaction of surface isocyanates with NO.
2448 CONCLUSIONS Vacuum TPD experiments with FCC catalysts coked with aniline, pyrrole and pyridine show that HCN and NH3 are formed via pyrolysis of nitrogen containing carbonaceous deposits on deactivated FCC catalysts. The HCN emission follows the order of increasing basicity from pyrrole to aniline and pyridine. Molecules with the highest basicity, i.e., pyridine derivatives are released at the highest temperature from the catalyst surface. HCN, formed via cracking of nitrogen containing polyaromatic compounds is oxidized to NO at temperatures above 550~ This reaction is favored if Pt based CO combustion promoters are present in the inventory. HCN and NH3 are formed via cracking and hydrolysis of nitrogen containing polyaromatic coke molecules. Both reduced nitrogen intermediates are released from coke species derived from aniline, pyridine and pyrrole cracking. Nitrogen and carbon containing species in the coke are oxidized sequentially during the regeneration process (C: 450-700~ N: > 650~ NO can be reduced in situ with CO or NH3/HCN. - MCM-36 type additives with mixed alkaline earth aluminium oxide (MgO-A1203-MCM-36) pillars are highly active additives for the reduction of NO with CO under reaction conditions similar to the oxygen depleted zone of the FCC regenerator. The reaction is concluded to proceed via nitrite, nitrate and isocyanate intermediates which are adsorbed on the basic mixed oxide clusters in the interlayer galleries. N2 and N20 are formed on the catalyst by the reaction of isocyanates with NO. At temperatures characteristic of the FCC regeneration process N20 decomposes over basic oxide clusters in the composite materials yielding N2. Consequently, the reduction of NO by CO over MCM-36 type materials might be explained as a two-step process involving the formation of nitrous oxide as an intermediate. The additives show a reduction of NOx emissions even in experiments simulating the regeneration of industrial coked FCC catalysts in a fluidized bed reactor. In contrast to Pt based additives, the nanocomposite materials do not oxidize reduced nitrogen containing intermediates (NH3, HCN) which are generated during the pyrolysis of nitrogen coke species, but lower the concentration of NOx released in the flue gases. It is speculated that during the regeneration of spent FCC catalysts the MCM-36 type additives catalyze, in addition to the NO + CO reaction, the SCR reaction of NO with NH3 to N2 on Bronsted acid sites in the zeolite layers or on the pillars in the interlayer galleries. HCN, which is a key intermediate in the nitrogen chemistry of the FCC unit might be hydrolyzed to NH3 over the basic oxide pillars in the interlayer galleries. The additives can be used in combination with Pt based CO promoters to simultaneously control the level of NO and CO in the regeneration unit. REFERENCES 1. R.H. Harding, A. W. Peters, J. R. D. Nee, Applied Catalysis A: General 2001,221,389. 2. W.-C. Cheng, G. Kim, A. W. Peters, X. Zhao, K. Rajagopalan, M. S. Ziebarth, C. J. Pereira, Catal. Rev.Sci. Eng. 1998, 40, 39. 3. J.-O. Barth, J. Kornatowski, J. A. Lercher, J. Mater. Chem. 2002, 12, 369. 4. E . A . Efthimiadis, E. F. Iliopoulou, A. A. Lappas, D. K. latrides, I. A. Vasalos, Ind. Eng. Chem. Res. 2002, 41, 5401. 5. J.-O. Barth, A. Jentys, J. A. Lercher, in Proceedings of the 17th Worm Petroleum Congress, Rio de Janeiro, Vol. 3; ISBN 0 85293 366 5, Institute of Petroleum, London, UK, 2003, pp. 445. 6. J.-O. Barth, A. Jentys, J. A. Lercher, submitted for publication 2003. 7. X. Zhao, A. W. Peters, G. W. Weatherbee, Ind. Eng. Chem. Res. 1997, 36, 4535. 8. J.-O. Barth, R. Schenkel, J. Kornatowski, J. A. Lercher, Stud. Surf Sci. Catal. 2001, 135, 136. 9. W.J. Roth, C. T. Kresge, J. C. Vartuli, M. E. Leonowicz, A. S. Fung, S. B. McCullen, Stud. Surf Sci. Catal. 1995, 94, 301.
2441
D E V E L O P M E N T OF NOVEL CATALYTIC ADDITIVES FOR THE I N S I T U REDUCTION OF NOx F R O M FLUID CATALYTIC C R A C K I N G UNITS Barth, J., Jentys, A. and Lercher, J.A. Technische Universit~itMtinchen, lnstitut ~r Technische Chemie, Lehrstuhl II 85747 Garching bei Mtinchen, Germany. E-mail: j [email protected]
ABSTRACT The chemistry during the FCC regeneration process was investigated providing a deep insight into the chemistry leading to the formation of NOx in the regenerator. From coke loaded spent FCC catalysts NH3 and HCN were determined as reduced intermediates for the formation of NOx. The improved understanding of such reaction intermediates formed during the FCC regeneration process was used to develop catalytic nanocomposite additives (derivatives of MCM-36) that promote the reduction of NO to N2. A novel approach to the synthesis of such catalysts was chosen, using layers (thickness: 2.5 nm) of zeolite MCM-22 spaced apart by clusters of MgO-A1203. The nanocomposite catalysts were tested in catalytic reactions under the conditions of the FCC process, demonstrating a significant reduction of NOx emissions.
INTRODUCTION Fluid catalytic cracking (FCC) is a key process in modern refineries. [1] Worldwide approximately 300 FCC units are operated, converting vacuum gas oil and high boiling residues into lighter fuel products and petrochemical feedstock. Due to its central function in modern integrated refineries, a range of technological improvements has been implemented, to increase the economical benefits from FCC units. In addition to investments concerning the process design, new catalysts and additives have been developed to fulfill the economic demands of the market. However, refiners are bound to invest also in eco-efficient technologies for the production of fuels and petrochemicals with significantly reduced emissions of environmental pollutants. [1] This is imposed by various stringent national and international regulations addressing emissions from a range of refinery processes and especially FCC regenerators, such as NO, SOx, CO and CO2 emissions from regenerator flue gases, t21 Approximately 2000 t/yr NOx are released from a typical refinery. The FCC units contribute to approximately 50 % of that. The concentrations of the NOx emissions from regenerator flue gases vary in the range of 50-500 ppm depending on the nature of the feed, the operating conditions of the FCC unit and the amount of CO promoter added. In the fluid catalytic cracking process, nitrogen containing species in the feedstock are cracked in the riser reactor to lighter molecules, while a fraction is deposited in the coke on the spent catalyst, During oxidative regeneration of the catalyst, more than 90% of the coke bound nitrogen is converted to molecular nitrogen (N2), while the rest is released in the form of NO. Sources of nitrogen leading to NOx formation are mainly the FCC feedstock ("fuel NOx"), while only minor amounts (< 10 ppm) are formed by N2 oxidation ("thermal NOx") and the reaction between radicals ("prompt NOx") in the regenerator. Over the last 20 years additives (Pt based CO combustion promoters) have been used to control CO emissions. However, these additives have significantly increased NO emissions. It has been speculated that such combustion promoters can oxidize reduced nitrogen containing intermediates such as HCN and NH3 to NOx. The reduction of NOx emissions from FCC units can be achieved by (i) using conventional end-of-the pipe technologies, such as the SCR (selective catalytic reduction) DeNOx process using NH3 as reducing agent, or (ii) by using additives to the FCC catalyst. These additives should catalyze the in situ reduction of NOx under the typical reaction conditions of the FCC unit. For the development of such additives it is essential to understand the reaction mechanisms, which lead to the formation of N2, NOx and reduced nitrogen containing intermediates such as HCN and NH3 under the conditions of the FCC process. In this paper we will discuss possible pathways of nitrogen containing species in the FCC unit. Especially, the nature of reduced nitrogen intermediates for the formation of NOx and factors controlling the formation of NOx from these intermediates were addressed. Novel nanocomposite materials based on
2442
MCM-22 (derivatives of MCM-36 with mixed oxide-pillars) were developed as NOx reduction additives enabling the reduction of NO with CO and reduced intermediates such as NH3 and HCN. The catalytic performance of MgO-A1203-MCM-36 was investigated under reaction conditions similar to those in the regenerator of the FCC unit. The catalysts were investigated under reaction conditions approximating the oxygen depleted and the oxygen rich zone of an FCC regenerator operating under full burn conditions. Oxygen deficient reaction conditions are encountered in the oxygen depleted zones of the dense and the diluted phase of the FCC fluidized bed. The incomplete combustion of coke leads to the formation of a reductive atmosphere in parts of the dense phase of the fluidized bed mainly due to the formation of CO and some hydrocarbons not desorbed in the stripper. Simultaneously, in the bottom region of the regenerator near the air inlet an oxygen-rich zone is encountered. The MCM-36 type materials were tested as catalysts for the reduction of NO with CO as well as in regeneration experiments mixed with spent FCC catalysts in a fluidized bed reactor. The reaction mechanism that leads to the reduction of NO by CO to N2 was studied by IR spectroscopy.
EXPERIMENTAL The catalysts were prepared as described elsewhere. TM Catalytic experiments were performed in a fixed bed and fluidized bed reactor simulating the conditions of the FCC regeneration unit. [4] A detailed description of the physicochemical characterization of coked FCC catalysts and reaction intermediates formed during the regeneration can be found in reference [51 and [6]. R E S U L T S AND D I S C U S S I O N
Pathways of nitrogen in the FCC unit As we have shown earlier, polyaromatic pyrrole derivatives (alkylcarbazoles, alkylbenzocarbazoles, alkylindoles) are the main source for nitrogen in the feedstock of FCC units, ts] These molecules dominate over 6-ring nitrogen species (pyridine derivatives) like alkylquinolines and alkyltetrahydroquinolines. After cracking 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 LD-/MALDI-TOF-MS spectroscopy as main nitrogen containing components of the coke. The majority of these species possess relatively high molecular masses (m/e = 350-850) and are probably trapped in the meso/macropores of the FCC catalyst microspheres. We have shown by TPO and LD/MALDI-TOF-MS experiments that during oxidative regeneration these large, pre-graphitic type compounds are converted into smaller carbazole and quinoline type molecules. 161With increasing oxidation temperature smaller nitrogen containing coke species accumulate on the catalyst surface. They are oxidized only after most of the non-heteroatomic hydrocarbons have been burned off. Such pyrrole and pyridine derivatives are probably trapped in that stage of regeneration on the strong Bronsted acid sites of the cracking catalyst. Such coke species are the actual precursors for the formation of N2 and NOx in the FCC regeneration unit (Fig. 1).
N 2 (g0 % of co ke IV)
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735 ~"0 --~EGENERATO 720"C
30 m
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Figure 1. Pathways of nitrogen containing compounds in a typical FCC unit.
2443
V a c u u m T P D m e a s u r e m e n t s of F C C catalysts c o k e d wi t h m o d e l s u b s t a n c e s b e a r i n g n i t r o g e n f u n c t i o n a l grou p s (aniline, pyrrole and pyridine) Temperature programmed desorption experiments in vacuum have been performed with FCC catalysts that had been coked with aniline, pyrrole or pyridine. The desorption of NH3 and HCN from the samples was followed by mass spectroscopy in order to compare the effect of different nitrogen containing precursor, species in the coke on the formation of reduced intermediates which are possible sources for the formation of NOx. Figure 2 shows the desorption of HCN (m/e = 27) and NH3 (NH +, m/e =15) from the samples. The maximum of HCN emissions follows the order of increasing basicity from pyrrole (maximum at 777~ to aniline (791~ and pyridine (813~ Pyridine derivatives in the coke are removed last from the acid sites of the FCC catalyst and form HCN. The comparison of the formation of NH3 from the coked catalysts shows a broad maximum a t - 7 1 6 ~ for the FCC catalyst coked with aniline. For the pyrrole and pyridine coked samples lower amounts of NH3 have been detected. Again, for the sample deactivated with pyridine the formation of reduced nitrogen species occurred at higher temperatures than for the less basic pyrrole. It can be speculated that in the polyaromatic coke formed by aniline cracking a higher fraction of amino (-NH2) groups is present which form ammonia upon pyrolysis of the carbonaceous deposits. The comparison of the desorption curves in Figure 2 indicates that the emission of HCN occurs at higher temperatures than that of NH3 (pyridine: 813 vs. 7 5 0 ~ 2 . 5 0 E -07
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Figure 3. Temperature programmed desorption of species from a coked FCC catalyst from a commercial FCC unit (left: p = 10.3 mbar, right: p = 1 bar, 100 ml He/min, 0 % 02, detailed view). V a c u u m T P D m e a s u r e m e n t s o f c o k e d F C C catalysts Figure 3 (left) shows the desorption of species from a coked FCC catalysts under vacuum (p = 10 -3 mbar) in a temperature range of 50-700~ The maximum of H20 desorption was observed at temperatures between 100-200~ H2 was formed during pyrolysis at temperatures above 450~ (not shown), with a maximum at 870~ and HCN desorption reached a maximum at 840~ For the identification of NH3 emitted
2444 from the catalyst the signal m/e = 15 was followed. The maximum at 620~ cannot be attributed unambiguously to NI-I+ as at the same temperature a maximum for the signals m/e = 13 and 14 can be observed hinting at methyl fragments (CH3 +) desorbing from the catalyst. However, at temperatures of -800~ the desorption curve of m/e = 15 shows a shoulder which can be attributed to NH +. This shoulder is more intense (maximum: 820~ if the experiment is performed in flowing He (100 ml/min) (cf. Fig. 3, right).
Temperature programmed oxidation experiments with coked FCC catalysts: the effect of Pt-based combustion promoters TPO experiments were carried out with samples of a coked FCC catalyst (232 ppm nitrogen; flow 100 ml He/min; 5 % 02). As it can be seen in Figure 4 carbon and nitrogen containing species are burnt sequentially. The maximum of NO formation (653 ~ occurred at significant higher temperatures than that of CO2 (570~ and CO (parallel to CO2; not shown). Figure 5 shows the formation of NO, NO2 and HCN from oxidation of carbonaceous deposits in the presence of a CO combustion promoter. In the absence of such an additive, the maximum of NO emission was observed at higher temperatures than that of HCN (653 vs. 622~ Note that NO2 is formed at lower temperatures (570~ occurring together with the maximum of CO2 formation, while NH3 was not detected in the presence of oxygen in these experiments. After addition of l wt.% commercial CO combustion promoter (Pt based additive) significant higher amounts of NO and NO2 were formed and the maximum of desorption was shifted to lower temperatures [611 vs. 653~ (NO), 558 vs. 570~ (NO2)]. The addition of the CO combustion promoter leads to an almost complete oxidation of HCN and a simultaneous increase of NO and NO2 (not shown) concentrations, of which the increase for NO2 is less pronounced. Higher oxygen concentrations in our TPO experiments, as well as higher excess oxygen in the flue gas stream of the FCCU regenerator are correlated with higher NOx emissions. It was demonstrated earlier by Zhao et al. that NOx is not formed from the oxidation of molecular N2 from the air ("thermal NOx"), but results from the oxidation of reaction intermediates such as HCN and NH3 from coke pyrolysis. [7] The present results indicate that in a typical full combustion regenerator most of reduced nitrogen species (HCN and NH3) are oxidized to NO as long as there is an excess of oxygen. Under partial combustion conditions a significant amount of NH3 and HCN should be present in the FCC regenerator, which can react then subsequently to N2 or NO. CO combustion promoters favor the formation of NO by (i) oxidizing intermediates such as HCN and NH3 to NO and by (ii) reducing the concentration of CO which can act as reductant for NO. 8.00E-10 ~ 5 6.00E-10 ~.)
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Figure 4. Temperature programmed oxidation (TPO) of a coked FCC catalyst (5 % 02).
Materials The above results clearly demonstrate that an in situ reduction of NOx emissions from FCC units can be only achieved by the reaction of NO with (i) CO or (ii) NH3 / HCN formed during the pyrolysis of nitrogen containing coke species. The use of nanocomposite materials based on zeolite MCM-22 with mixed alkaline earth oxide pillars between the zeolite layers is a novel approach to enable both reactions (over basic oxide clusters in the interlayer galleries and Bronsted acid sites in the zeolitic sheets). In contrast to noble metal containing catalysts such additives should not increase NOx emissions by the oxidation of reduced intermediates such as NH3 and HCN.
2445 Table 1 summarizes the textural properties and the acid site concentrations (determined from temperature programmed desorption of ammonia I81 of MgO-A1203-MCM-36 investigated in this study. A detailed physicochemical characterization and investigation of the acid-base character of such nanocomposite materials can be found in reference t31. Table 1. Properties of MgO-A1203-MCM-36 investigated in this study. Sample
Si [wt.%]
Al [wt.%]
Na [wt.%]
Mg [wt.%]
MgO-A1203-MCM-36
27.92
21.28
< 0.10
0.64
BET s.a. [m2/g] 348
c (acid site) (mmol/g) 0.91
TEM micrographs demonstrate clearly that the materials have a layered structure. The pillared zeolite sheets form aggregates of appr. 100-150 nm length and -100 nm width. The layer thickness can be estimated to be appr. 2.5 nm which is consistent with the values reported by Roth et al. [9] The presence of pillars keeping the layers apart cannot be observed directly probably due to insufficient contrast. These less dense areas between the sheets are attributed to interlayer distances being equal to ca. 0.1 nm (Fig. 5 right). The packing of the MCM-22 sheets seems to be less regular on the borders and the outer termination of the aggregates (Fig. 5 left).
Figure 5. Transmission electron micrographs of MgO-A1203-MCM-36. Kinetic m e a s u r e m e n t s MgO-A1203-MCM-36 was tested as catalysts for the reduction of NO with CO under reaction conditions simulating the FCC regeneration process. CO is the dominating reducing agent in the regenerator, due to the incomplete combustion of coke. Figure 6 shows an example for the reduction of NO with CO over MgO-AlzO3-MCM-36 under conditions similar to the oxygen depleted (Fig. 6a) and rich (Fig. 6b) phase of the FCC regenerator operated under full burn conditions. N2, NO2 and N20 were formed as reaction products, while CO2 was generated by the oxidation of CO. In the relevant temperature region of 600-750~ high yields (50-84 %) of N2 were obtained under reaction conditions similar to the oxygen depleted dense and diluted phase of the fluidized bed of the FCC regenerator (0.5 % 02). In the presence of higher oxygen concentrations (2.4 % 02), which are encountered in the lower parts of the dense phase, significantly lower yields (10-40 %) of N2 were measured. In the first case considerable amounts of N20 were formed with a maximum yield of 50 % at 575~ whereas under the reaction conditions with high oxygen concentration only 17 % N20 was formed with its maximum shifted to higher temperatures (625~ In contrast to this observation, under oxygen rich conditions (2.4 % 02) higher yields of NO2 were obtained. For both cases the conversion of CO and yield of CO2 increased with rising temperature and reached a constant maximum degree of conversion at 575-600~
2446
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Figure 6. NO reduction with CO over MgO-A1203-MCM-36" a) 700 ppm NO, 1.4 % CO, 0.5 % 02; b) 1000 ppm NO, 4.4 % CO, 2.4 % 02. MCM-36 type additives with mixed alkaline oxide aluminium pillars (MgO-AI203; BaO-A1203) show a reduction of NOx emissions (-~20 %) even in experiments simulating the regeneration of industrial coked FCC catalysts in a fluidized bed reactor (a detailed discussion can be found in reference t61). In contrast to Pt based additives, the MCM-36 derivative does not oxidize reduced nitrogen containing intermediates (NH3, HCN), which are generated during the pyrolysis of nitrogen coke species, but lowers the concentration of NOx released in the flue gases. It is speculated that during the regeneration of spent FCC catalysts the MCM36 type additives catalyze, in addition to the NO + CO reaction, the SCR reaction of NO with NH3 to N2 on Bronsted acid sites in the zeolite layers or on the pillars in the interlayer galleries. HCN, which is a key intermediate in the nitrogen chemistry of the FCC unit might be hydrolyzed to NH3 over the basic oxide pillars in the interlayer galleries. The additives can be used in combination with Pt based CO promoters to simultaneously control the level of NO and CO in the regeneration unit.
IR spectroscopy: adsorption of NO, 02 and CO on MgO-A12Os-MCM-36 For the investigation of intermediate species formed on the surface during the reduction of NO with CO in the presence of oxygen co-adsorption of NO and 02 followed by adsorption of CO was carried out. The formation of nitrite and nitrate species (bridged bidentate nitrate, monodentate nitrate on AI, linear nitrite on AI) was observed. Carbonates (from adsorption of air CO2) on the basic material were removed upon NO adsorption. After adsorption of 1000 ppm NO in the presence of 3 % 02 predominantly nitrate species were formed on the nanocomposite materials. Bands at 1575 cm -~ characteristic of monodentate nitrates on A1 and 1552 cm -1 (bidentate nitrate on AI close to Mg) were observed, whereas nitrites (1459, 1463 cm-l: linear nitrite on A1) were only detected in minor concentrations under oxidative conditions. After adsorption of NO and 02 the sample was equilibrated with 5 % CO in flowing He. Figure 7 shows differences in the IR spectra measured during heating of the catalyst from 200 to 500~ in the presence of CO. Bands indicative of nitrates and nitrites decreased in intensity (negative bands) while bands at 1378, 1384 1588, 1610 and 1637 cm -~ first increased in intensity, went through a maximum a t - 3 5 0 ~ and decreased again with increase in temperature. Partially overlapped bands were measured above 250~ at -2280-2240 and 2227 cm -I which with rising temperature increased continuously in intensity. The first set of IR bands can be attributed to bicarbonate (1445, 1646 cm -1, shoulder at 3610 cm-I), uni- (1410, 1540-1580 cm -1) and bidentate (1378 cm -I) carbonate species adsorbed on basic sites of the MgO-A1203-MCM-36 material. Bands at 2930 (not shown) 1610 and 1384 cm -1 indicated the presence of formate species on the MgO-A1203 clusters. The broad bands at 2280-2240 cm -~ are indicative of the degenerate stretching vibration of surface isocyanate species adsorbed on the MgO-AI203 pillars (AI-NCO: 2260, Mg-NCO: 2240 cm-1). The narrow peak centered at 2227 cm-1 is tentatively ascribed to the N-N stretching vibration of N20 formed during the reaction.
2447 I"--O(30 O3 CD C0 t,.D ~ I,~
C3 CO 03
.005
U C r
o
(oO ..Q
13 200
-.005
300~ 400 .~. 500 ~ 1700 1600 1500 1400 1300 W a v e n u m b e r [crrrl]
Figure 7.Differences of IR spectra measured during reaction of 5 % CO on MgO-A1203-MCM-36 after exposure of the catalyst to 1000 ppm NO and 5 % 02. The results demonstrate that MgO-A1203-MCM-36 type materials containing mixed alkaline earth aluminium oxide pillars show high catalytic activity in the reduction of NO with CO, in the presence of oxygen, to N2 and CO2. Generally, the conversion of NO and yields of N2 and N20 were significantly higher under oxygen deficient reaction conditions, which are encountered in the oxygen depleted zones of the dense and the diluted phase of the FCC fluidized bed. The presence of higher oxygen concentrations inhibits the reduction of NO by CO as the CO/O2 reaction is favoured in expense of the CO/NO reaction. Under reaction conditions similar to the oxygen-rich zone, which is encountered in the bottom region of the regenerator near the air grid (500-625~ oxygen excess), NO is mainly converted to NO2 over the MCM-36 derivative. As a direct oxidation of NO to NO2 is not favoured thermodynamically at such high temperatures, high yields of NO2 can only be attributed to the decomposition of surface nitrite and nitrate species formed on the basic oxide clusters (MgO-AI203) in the interlayer galleries. Note that the temperature programmed desorption of NO2 from MgO-A1203-MCM-36 has shown that such ionic NOx species still decompose in the hightemperature region above 500~ Various types of surface nitrite and nitrate species (mono-, bidentate, bridged nitrates) have been observed by IR spectroscopy on the basic MCM-36 type materials. The interaction of NO with the pillared zeolites led to the removal of carbonate species present on the basic oxide clusters and the formation of different ionic NOx surface species, which are probably located on the pillars between the zeolite layers. Interestingly, nitrates were already formed in the absence of oxygen, suggesting an oxidation of NO by reactive oxygen species present in the mixed oxide clusters in the interlayer galleries. In the presence of 3 % oxygen mainly nitrate species were observed on the surface with bidentate nitrates on A1 close to Mg (1552 cm -1) being the dominant species. Our IR measurements showed that with increasing temperature the presence of CO (after exposure of the catalyst to NO and 02) led to a removal of nitrite and nitrate species from the catalyst surface. Simultaneously, formate and various (bi-)carbonate species were observed, which can only result from an oxidation of CO during the reaction with the surface NOx species. The concentration of carbonate and formate species (adsorbed on basic sites) increased on the catalyst surface u n t i l - 350 ~ before it started to decrease again, which can be explained by a thermal desorption of the weakly bound species. For the understanding of the reaction mechanism of NO reduction with CO over MgO-A1203-MCM-36 it is important to recognize that at temperatures above 300~ a distinct formation of isocyanate species and N20 was observed. The wavenumbers of the partially overlapped IR bands (2260, 2240 cm 1) suggest that the -NCO intermediates are most probably adsorbed on the MgO-A1203 pillars (A1NCO; Mg-NCO). Simultaneously with the observation of isocyanate species the formation of N20 was indicated by the intense band of the N20 stretching vibration at 2227 cm -1. Isocyanates such as A1-NCO and Mg-NCO species may be formed by the reaction of nitrite and nitrate species with CO. The formation of the N-N bond in the reduction of NO with CO leading to N2 and N20, is tentatively ascribed to the reaction of surface isocyanates with NO.
2448 CONCLUSIONS Vacuum TPD experiments with FCC catalysts coked with aniline, pyrrole and pyridine show that HCN and NH3 are formed via pyrolysis of nitrogen containing carbonaceous deposits on deactivated FCC catalysts. The HCN emission follows the order of increasing basicity from pyrrole to aniline and pyridine. Molecules with the highest basicity, i.e., pyridine derivatives are released at the highest temperature from the catalyst surface. HCN, formed via cracking of nitrogen containing polyaromatic compounds is oxidized to NO at temperatures above 550~ This reaction is favored if Pt based CO combustion promoters are present in the inventory. HCN and NH3 are formed via cracking and hydrolysis of nitrogen containing polyaromatic coke molecules. Both reduced nitrogen intermediates are released from coke species derived from aniline, pyridine and pyrrole cracking. Nitrogen and carbon containing species in the coke are oxidized sequentially during the regeneration process (C: 450-700~ N: > 650~ NO can be reduced in situ with CO or NH3/HCN. - MCM-36 type additives with mixed alkaline earth aluminium oxide (MgO-A1203-MCM-36) pillars are highly active additives for the reduction of NO with CO under reaction conditions similar to the oxygen depleted zone of the FCC regenerator. The reaction is concluded to proceed via nitrite, nitrate and isocyanate intermediates which are adsorbed on the basic mixed oxide clusters in the interlayer galleries. N2 and N20 are formed on the catalyst by the reaction of isocyanates with NO. At temperatures characteristic of the FCC regeneration process N20 decomposes over basic oxide clusters in the composite materials yielding N2. Consequently, the reduction of NO by CO over MCM-36 type materials might be explained as a two-step process involving the formation of nitrous oxide as an intermediate. The additives show a reduction of NOx emissions even in experiments simulating the regeneration of industrial coked FCC catalysts in a fluidized bed reactor. In contrast to Pt based additives, the nanocomposite materials do not oxidize reduced nitrogen containing intermediates (NH3, HCN) which are generated during the pyrolysis of nitrogen coke species, but lower the concentration of NOx released in the flue gases. It is speculated that during the regeneration of spent FCC catalysts the MCM-36 type additives catalyze, in addition to the NO + CO reaction, the SCR reaction of NO with NH3 to N2 on Bronsted acid sites in the zeolite layers or on the pillars in the interlayer galleries. HCN, which is a key intermediate in the nitrogen chemistry of the FCC unit might be hydrolyzed to NH3 over the basic oxide pillars in the interlayer galleries. The additives can be used in combination with Pt based CO promoters to simultaneously control the level of NO and CO in the regeneration unit. REFERENCES 1. R.H. Harding, A. W. Peters, J. R. D. Nee, Applied Catalysis A: General 2001,221,389. 2. W.-C. Cheng, G. Kim, A. W. Peters, X. Zhao, K. Rajagopalan, M. S. Ziebarth, C. J. Pereira, Catal. Rev.Sci. Eng. 1998, 40, 39. 3. J.-O. Barth, J. Kornatowski, J. A. Lercher, J. Mater. Chem. 2002, 12, 369. 4. E . A . Efthimiadis, E. F. Iliopoulou, A. A. Lappas, D. K. latrides, I. A. Vasalos, Ind. Eng. Chem. Res. 2002, 41, 5401. 5. J.-O. Barth, A. Jentys, J. A. Lercher, in Proceedings of the 17th Worm Petroleum Congress, Rio de Janeiro, Vol. 3; ISBN 0 85293 366 5, Institute of Petroleum, London, UK, 2003, pp. 445. 6. J.-O. Barth, A. Jentys, J. A. Lercher, submitted for publication 2003. 7. X. Zhao, A. W. Peters, G. W. Weatherbee, Ind. Eng. Chem. Res. 1997, 36, 4535. 8. J.-O. Barth, R. Schenkel, J. Kornatowski, J. A. Lercher, Stud. Surf Sci. Catal. 2001, 135, 136. 9. W.J. Roth, C. T. Kresge, J. C. Vartuli, M. E. Leonowicz, A. S. Fung, S. B. McCullen, Stud. Surf Sci. Catal. 1995, 94, 301.