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TEOM studies on the adsorption of p-xylene in coked FCC catalysts: observation of coke promoting chemical reaction
Applied Catalysis A: General 274 (2004) 269–274 www.elsevier.com/locate/apcata
TEOM studies on the adsorption of p-xylene in coked FCC catalysts: observation of coke promoting chemical reaction Chi Keng Lee, Lynn F. Gladden, Patrick J. Barrie* Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB23RA, UK Received in revised form 5 July 2004; accepted 6 July 2004 Available online 7 August 2004
Abstract A tapered element oscillating microbalance (TEOM) was used to measure the rates of adsorption and desorption of p-xylene from FCC catalyst at 473 K. The original aim of the work was to investigate whether coke in the catalyst, formed by an earlier reaction with isopropanol at 773 K, influenced the adsorption kinetics. It was actually found that p-xylene reacted in the coked catalyst, even though no reaction occurred in the fresh catalyst at this temperature. This is a rare direct observation of coke causing chemical reaction in a system in which no reaction occurs in the absence of coke. # 2004 Elsevier B.V. All rights reserved. Keywords: Adsorption; Coke; FCC catalyst; Microbalance; TEOM
1. Introduction The fluid catalytic cracking (FCC) process on an oil refinery is a key unit operation in which heavy hydrocarbon molecules are cracked to form smaller more valuable products. The FCC catalyst particles employed contain zeolite crystallites, most commonly rare-earth exchanged zeolite Y, embedded in a matrix that contains alumina, clay and sometimes silica. The zeolite component is responsible for the high activity and selectivity of FCC catalysts. The matrix material has some catalytic activity (e.g. in cracking molecules too large to enter the zeolite) and it provides attrition resistance. It also acts as a heat sink and can reduce the extent of heavy-metal poisoning of the zeolite component [1–3]. There have been a large number of studies on the catalytic performance of zeolites for cracking reactions. For instance, these have explored the influence of pore structure and Si/Al ratio, characterized the different types of acid site that may be present, and proposed reaction mechanisms [4–8]. One particularly important topic that has been studied is the * Corresponding author. Tel.: +44 1223 331864; fax: +44 1223 334796. E-mail address: [email protected] (P.J. Barrie). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.010
effect of coke on the activity and selectivity of zeolite-based catalysts [8–13]. Coke can influence catalytic activity through both site poisoning and pore blocking mechanisms, and it is important to distinguish between the two. For instance, this can be done by measuring the diffusion coefficients of adsorbates within coked zeolites [14,15]. It is also important to know whether the coke is distributed homogeneously throughout the zeolite particles. For instance, coke may preferentially form on the external surface of zeolites crystallites, or will only be produced at the outer edge of the zeolite crystallites if the reaction is diffusion limited [16,17]. Molecular transport within zeolites has been extensively studied because it can influence overall reaction rates, and also because of the use of zeolites in separations processes [18,19]. In a recent paper, we described experiments that used a tapered element oscillating microbalance (TEOM) to measure the rates of adsorption and desorption of n-hexane, n-heptane, n-octane, toluene and p-xylene from catalysts under non-reacting conditions [20]. The catalysts studied were a pure rare-earth exchanged zeolite Y and a commercial FCC catalyst. It was found that the rates of adsorption and desorption were the same for the FCC catalyst as for the pure zeolite Y sample, indicating that mass transport in the
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matrix component of the FCC catalyst is rapid and not a limiting step in the kinetics of adsorption for the hydrocarbons studied. Detailed modelling and numerical simulation of the results was performed and these indicate that the limiting step governing the adsorption kinetics was local transport of molecules at the zeolite–matrix or zeolite– vapour interface [21]. Previous work measuring the adsorption rate of methanol in zeolite NaX found that the apparent surface resistance to adsorption changed during successive measurements; this was attributed to a gradual build-up of coke on the crystal surface which then affected the adsorption kinetics [22,23]. We therefore decided to investigate whether the presence of coke influenced the adsorption and desorption kinetics of pxylene from the FCC catalyst which we had previously studied in detail. As it turned out, we discovered an unusual instance of coke promoting a chemical reaction that would not occur in its absence. While it is known that coke on metal surfaces can act in a beneficial way, this is usually because it enhances product selectivity rather than catalyst activity [24,25]. There are, however, some examples where adsorption and chemical reaction are believed to proceed on the carbonaceous overlayer rather than on the metal itself [26,27]. In the case of reactions in zeolites, several groups of workers have inferred that coke plays an active part in the reaction [28–32]. However, it is rare to observe directly a chemical reaction with coke, and the current work is an example of this phenomenon in a system in which no reaction occurs if coke is absent. This paper first describes the coking of FCC catalyst by reaction with isopropanol vapour at 773 K. It then presents results on the adsorption and desorption of p-xylene vapour in both coked and uncoked catalyst at 473 K. Some experiments were also performed where the base quinoline was introduced into the catalyst prior to the experiments measuring p-xylene adsorption. Mass changes in the catalyst were monitored continuously using a TEOM, while a mass spectrometer was used to provide independent evidence of chemical reaction.
changes can be found by measuring the natural resonance frequency of a tapered quartz element containing the sample of interest [33]. It allows accurate mass measurements to be made rapidly at elevated temperatures under conditions in which the adsorbate vapour passes through the catalyst bed. The flow conditions used, and the fact that the amount of catalyst used is small (typically 60 mg), means that external mass and heat transfer limitations are far less significant than when using other microbalance techniques. Further, it is possible to measure mass in a TEOM every 0.1 s and so rapid changes can be investigated [20]. The composition of the inlet stream to the TEOM could be monitored using a gas chromatograph (GC) equipped with a flame ionization detector, while the composition of the outlet stream of the TEOM could be analysed using either GC or a time-of-flight mass spectrometer (MS). A schematic diagram of the experimental configuration is shown in Fig. 1. All gas flows in the apparatus were controlled by automatic mass flow controllers. The helium flowrates in all experiments were 200 ml/min at STP conditions. The function of the helium purge stream was to ensure that any hydrocarbons desorbing from the catalyst were removed from the balance. All pipes and valves were heated to ensure that no hydrocarbon condensation took place within them. During activation, the catalyst sample is exposed only to helium pretreat gas. However, switching valve 1 exposes the catalyst to the vapour of whichever hydrocarbon is present in the saturator (variously isopropanol, p-xylene and quinoline in this work). It is important that the needle valve on the vent outlet from valve 1 is adjusted to ensure that there is no pressure imbalance in the system when valve 1 is switched. Any pressure imbalance would cause misleading mass measurements as the TEOM is sensitive enough to detect changes in the density of gas within the element.
2. Experimental The commercial FCC catalyst was supplied by BP Oil. It contains rare-earth exchanged zeolite Y in a matrix, and consists of spherical particles of approximately 70 mm diameter. The catalyst was steamed at 1089 K for 5 h in order to cause ‘‘ultrastabilisation’’. This process causes dealumination of the zeolite component, but enhances the overall stability of the catalyst [1,2]. It was performed in order to give a catalyst with a composition and structure comparable to that found in FCC units, as the steaming mimics the actual conditions experienced in an FCC unit. The FCC catalyst was placed at the bottom of the quartz element of a Rupprecht and Patashnick PMA 1500 TEOM. The TEOM is a form of inertial balance in which mass
Fig. 1. Diagram of the experimental configuration used.
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Fig. 2. Summary of the different treatments experienced by the sample during the experiments. Sample mass was measured by the TEOM during each of the four experiments labelled 1–4.
A summary of the different experiments performed is shown in Fig. 2. The element resonance frequency, and thus sample mass, was monitored at all stages of this process, using a time resolution of 0.3 s per data point. For comparison purposes, some experiments were performed using inert quartz particles in the microbalance rather than FCC catalyst.
3. Results and discussion The main aim of reacting the FCC catalyst with isopropanol in this work was the formation of significant amounts of coke. Isopropanol is known to undergo dehydration to
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form propene within acidic zeolites [34,35]. Propene readily reacts further at elevated temperatures to produce a range of aliphatic hydrocarbons [36,37], some of which undergo cyclisation reactions leading to the formation of aromatic and polyaromatic species that ultimately become known as coke [38,39]. While it has been suggested that the initial stage of the propene reaction is direct protonation at a Brønsted acid site to give a carbenium ion [40], it is now believed that adsorbed propene actually forms an alkoxide species which undergoes rapid reaction with other hydrocarbon molecules at elevated temperatures [41–43]. Indeed carbenium ions are now more commonly treated as the highenergy transition states in the reaction mechanism rather than as intermediates [44]. Fig. 3 shows the mass changes occurring during the exposure of the FCC catalyst to isopropanol vapour for 1 h. The initial increase in mass at time t = 400 s is due to adsorption of isopropanol which reacts rapidly to give light hydrocarbon products. The gradual increase in mass over the next hour reflects the formation of heavy products within the catalyst. At time t = 4000 s, desorption is allowed to occur. Even after long desorption periods at 773 K, significant amounts of coke are retained within the catalyst; in the case of the results in Fig. 3, the coke content of the catalyst is 0.4 wt.%. Fig. 4 shows the mass spectrum obtained during the isopropanol experiment with FCC catalyst. For comparison purposes, the mass spectrum recorded with quartz particles in the microbalance at 773 K is also shown. When no catalyst is present, there is no reaction and the mass spectrum obtained corresponds well to that of pure isopropanol [45]. The weak peak at 59 mass units corresponds to M+–H, and the strong peak at 45 mass units corresponds to M+– CH3. With the FCC catalyst, no isopropanol is detected in the mass spectrum, indicating 100% conversion has been
Fig. 3. Mass measurements during the treatment of FCC catalyst with isopropanol at 773 K. At time t = 0 s, the catalyst is in a pure helium environment. Valve 1 is switched at time t = 400 s so that isopropanol (partial pressure 0.47 bar) flows through the catalyst bed. At time t = 4000 s, valve 1 is switched back again, and so desorption takes place in pure helium after this time.
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Fig. 4. Mass spectra obtained on the products when isopropanol is passed through (a) quartz particles, (b) FCC catalyst, at 773 K.
achieved, while the expected major products are present. Propene is responsible for the cluster of peaks between 37 and 42 and also for the peak at 27 mass units, while water gives the peak at 18 mass units [45]. There are also weak peaks at 55, 69 and 84 mass units. These may be assigned to hexene isomers and indicate that oligomerisation reactions are taking place. The catalyst that had been coked by the isopropanol reaction was then exposed to a 60 s pulse of p-xylene vapour at 473 K. The TEOM results are shown in Fig. 5, together with results from the same experiment on an uncoked FCC catalyst. It had been anticipated that the presence of coke might affect the kinetics of adsorption and desorption (e.g. by pore blocking or by affecting the surface barrier to transport at the particle surface). Instead, it is apparent that the rate of adsorption is the same in the coked and fresh catalyst. Surprisingly, though, there is a gradual increase in mass of the coked catalyst during the p-xylene pulse.
Further, even after long desorption times, the mass of the coked FCC catalyst does not return to its value before the pxylene pulse. These results imply that p-xylene must have reacted in the coked FCC catalyst sample and formed further coke within it. This is an interesting result, as no reaction occurred using the fresh catalyst under identical conditions. Fig. 6 shows mass spectra recorded during the p-xylene pulses on both the fresh and coked FCC catalyst samples. The mass spectrum with the fresh FCC catalyst corresponds to that expected for pure p-xylene [45]. The major peaks are at 106 and 91 mass units, with a weaker peak at 105 mass units. These correspond to M+, M+–CH3 and M+–H species, respectively. The mass spectrum obtained with the coked FCC catalyst is quite different. As well as peaks from unreacted p-xylene, it contains additional peaks at 133 and 148, a trace peak at 175, and a change in the relative intensity of the peaks at 105 and 106 mass units. This confirms that p-xylene has reacted with the coked catalyst, even though no reaction occurs in the absence of coke. The most likely product that is consistent with the mass spectrum is dimethylisopropylbenzene formed from alkylation of the p-xylene molecule. This would explain the peaks observed at 148 (M+), 133 (M+–CH3) and 105 (M+–CH3– C2H4) [45,46]. One can go further and speculate that a second alkylation step might occur to give dimethyldiisopropylbenzene as another product. This would have its strongest peak at 175 mass units, and a weak peak is observed at this position in the experimental mass spectrum. It is interesting that the apparent product of the reaction between the coke and p-xylene at 473 K comes from the addition of C3 species given that the coke was originally formed from reactive C3 species. It is very unlikely that there is any residual isopropanol present in the catalyst. Mass spectra showed that 100% conversion of isopropanol was achieved, and a period of 1 h in helium at 773 K should be more than adequate to ensure complete removal of isopropanol even if it had not all reacted. While it might be
Fig. 5. Mass measurements at 473 K for a 60 s pulse of p-xylene (partial pressure 0.058 bar) on fresh FCC catalyst and on FCC catalyst containing coke from the isopropanol reaction. For the sake of clarity, not all data points recorded by the TEOM are shown.
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Fig. 6. Mass spectra obtained during the p-xylene pulses at 473 K using (a) fresh FCC catalyst, (b) coked FCC catalyst.
envisaged that some isopropanol molecules could be trapped in the catalyst by steric blocking due to the presence of coke, such molecules would not themselves be accessible to a larger molecule such as p-xylene. The possibility of the chemical reaction being between residual isopropanol and pxylene can thus be excluded. It is also highly unlikely that C3 carbenium ions can be present in the sample given that they have been found not to be stable species in zeolite Y even at low temperatures [41–43]. The reaction at 473 K must therefore be between pxylene and the coke species formed during the isopropanol reaction. The coke present will be ‘‘high temperature’’ coke as it was formed at 773 K. Structural characterization of coke formed from propene and other reactants in steamed zeolite Y at high temperature have established that it consists mainly of polyaromatic molecules that contain four to six aromatic rings, and that these rings are likely to have alkyl
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substituents [47,48]. If the coke molecules have isopropyl substituents, which is plausible as they were formed from isopropanol and propene reactions, then a transalkylation reaction between p-xylene and coke might occur in a similar fashion to that envisaged during the liquid-phase isopropylation of naphthalene [28,29]. Such a reaction would still require the presence of acid sites. In order to test this hypothesis, additional experiments were performed in which quinoline was adsorbed onto the catalyst prior to exposure with p-xylene. It was found that no reaction occurs with pxylene at 473 K using a coked catalyst that had been exposed to quinoline. Thus the alkylation reaction with coke does indeed require acid sites to be present within the catalyst. Experiments were also performed using a series of pxylene pulses on the same coked FCC catalyst. The TEOM results for a series of six pulses, each of 1 min duration, are shown in Fig. 7. These show continued reaction between pxylene and coke during each pulse, with an increasing amount of residual carbonaceous material remaining in the catalyst after each pulse. The increase in mass detected, after allowing for desorption of p-xylene and products after each pulse is 0.024, 0.035, 0.047, 0.058, 0.065 and 0.068%, respectively. This small increase in mass does not necessarily reflect condensation reactions on the polyaromatic species already present, but may be due to the formation of ‘‘low temperature’’ coke [47] by the proposed alkylation reaction. Low temperature coke consists of molecules that are strongly adsorbed and not very volatile. For instance, it is unlikely that products resulting from more than one alkylation of a p-xylene molecule will be able to desorb completely from the catalyst at a temperature of only 473 K. Finally, it is worth commenting on whether coke affects the rates of adsorption and desorption of p-xylene at 473 K given that this was the original aim of the work. The rates of adsorption and desorption of p-xylene at 473 K on the catalyst that had been coked and then treated with quinoline
Fig. 7. Mass measurements at 473 K for a series of 60 s pulses of p-xylene on FCC catalyst containing coke from the isopropanol reaction.
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(so no reaction occurred) were the same as those measured on the fresh FCC catalyst. This observation shows that the limiting transport step affecting the adsorption kinetics of pxylene at 473 K is not affected by the presence of coke or quinoline at the loadings studied. This is in line with our previous conclusion that local transport at the zeolite–matrix interface is the limiting transport step [21].
4. Conclusions Coke formed by the reaction of isopropanol at 773 K in FCC catalyst is not an inert chemical species. Instead, it causes an alkylation reaction of p-xylene at 473 K to occur. There is no chemical reaction at this temperature in the absence of coke. The reaction in the coked catalyst requires the presence of acid sites to proceed as it was found that adsorption of the base quinoline caused loss of activity of the coked catalyst.
Acknowledgements We thank BP Oil and the Department of Trade and Industry, UK, for financial support of this work. We also thank Dr. Graham Ketley and Dr. Michael Hodges (BP Oil) for their help with this project.
References [1] P.B. Venuto, E.T. Habib Jr., Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekker, New York, 1979. [2] C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, New York, 1980. [3] J. Scherzer, Appl. Catal. 75 (1991) 1. [4] A. Corma, Catal. Lett. 22 (1993) 33. [5] J.A. Rabo, G.J. Gajda, Catal. Rev. Sci. Eng. 31 (1989) 385. [6] W.E. Farneth, R.J. Gorte, Chem. Rev. 95 (1995) 615. [7] A. Corma, A.V. Orchille´ s, Micropor. Mesopor. Mater. 35 (2000) 21. [8] K.A. Cumming, B.W. Wojciechowski, Catal. Rev. Sci. Eng. 38 (1996) 101. [9] J.B. Butt, E.E. Petersen, Activation, Deactivation and Poisoning of Catalysts, Academic Press, London, 1988. [10] S. Bhatia, J. Beltramini, D.D. Do, Catal. Rev. Sci. Eng. 31 (1989) 431. [11] H.G. Karge, Stud. Surf. Sci. Catal. 58 (1991) 531. [12] M. Guisnet, P. Magnoux, Appl. Catal. 54 (1989) 1. [13] H.C. Beirnaert, J.R. Alleman, G.B. Marin, Ind. Eng. Chem. Res. 40 (2001) 1337. [14] B. Paweewan, P.J. Barrie, L.F. Gladden, Appl. Catal. A: Gen. 167 (1998) 353.
[15] B. Paweewan, P.J. Barrie, L.F. Gladden, Appl. Catal. A: Gen. 185 (1999) 259. [16] L.G. Okkel, V.B. Fenelonov, V.N. Romannikov, L.P. Soloveva, Kinet. Catal. 33 (1992) 140. [17] J.-L. Bonardet, T. Domeniconi, P. N’Gokoli-Kekele, M.-A. SpringuelHuet, J. Fraissard, Langmuir 15 (1999) 5836. [18] J. Ka¨ rger, D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992. [19] N.Y. Chen, T.F. Degnan Jr., C.M. Smith, Molecular Transport and Reaction in Zeolites: Design and Application of Shape Selective Catalysts, VCH Publishers, New York, 1994. [20] C.K. Lee, S. Ashtekar, L.F. Gladden, P.J. Barrie, Chem. Eng. Sci. 59 (2004) 1131. [21] P.J. Barrie, C.K. Lee, L.F. Gladden, Chem. Eng. Sci. 59 (2004) 1139. [22] P. Grenier, F. Meunier, P.G. Gray, J. Ka¨ rger, Z. Xu, D.M. Ruthven, Zeolites 14 (1994) 242. [23] P. Grenier, V. Bourdin, L.M. Sun, F. Meunier, AIChE J. 41 (1995) 2047. [24] P.G. Menon, J. Mol. Catal. 59 (1990) 207. [25] G.C. Bond, Appl. Catal. A: Gen. 149 (1997) 3. [26] G.E. Vrieland, P.G. Menon, Appl. Catal. 77 (1991) 1. [27] A.A. Barresi, M. Cittadini, A. Zucca, Appl. Catal. B: Environ. 43 (2003) 27. [28] M. Guisnet, J. Mol. Catal. A: Chem. 182–183 (2002) 367. [29] G. Colon, I. Ferino, E. Rombi, P. Magnoux, M. Guisnet, React. Kinet. Catal. Lett. 63 (1998) 3. [30] Z. Da, P. Magnoux, M. Guisnet, Catal. Lett. 61 (1999) 203. [31] P. Andy, N.S. Gnep, M. Guisnet, E. Benazzi, C. Travers, J. Catal. 173 (1998) 322. [32] B. Arstad, B.S. Kolboe, J. Am. Chem. Soc. 123 (2001) 8137. [33] H. Patashnick, E.G. Rupprecht, Res. Dev. 28 (1986) 74. [34] E.A. Lombardo, J.M. Dereppe, G. Marcelin, W.K. Hall, J. Catal. 114 (1988) 167. [35] C.P. Bezoukhanova, Y.A. Kalvachev, Catal. Rev. Sci. Eng. 36 (1994) 125. [36] A.K. Ghosh, R.A. Kydd, J. Catal. 100 (1986) 185. [37] F. Geobaldo, G. Spoto, S. Bordiga, C. Lamberti, A. Zecchina, J. Chem. Soc., Faraday Trans. 93 (1997) 1243. [38] P. Magnoux, F. Machado, M. Guisnet, W.C. Conner, H. Schulz, N.A. Prokudina, S. Csicsery, P.G. Menon, J.P. Fraissard, Stud. Surf. Sci. Catal. 75 (1993) 435. [39] K. Moljord, P. Magnoux, M. Guisnet, Appl. Catal. A: Gen. 122 (1995) 21. [40] M.C. Grady, R.J. Gorte, J. Phys. Chem. 89 (1985) 1305. [41] J.F. Haw, B.R. Richardson, I.S. Oshiro, N.D. Lazo, J.A. Speed, J. Am. Chem. Soc. 111 (2052) 1989. [42] J.F. Haw, T. Xu, Adv. Catal. 42 (1998) 115. [43] A. Bhan, Y.V. Joshi, W.N. Delgass, K.T. Thomson, J. Phys. Chem. B 107 (2003) 10476. [44] V.B. Kazansky, Catal. Today 51 (1999) 419. [45] NIST Chemistry WebBook. http://webbook.nist.gov/. [46] D.H. Williams, I. Fleming, Spectroscopic Methods in Organic Chemistry, McGraw-Hill, London, 1995. [47] M. Guisnet, P. Magnoux, Appl. Catal. A: Gen. 212 (2001) 83. [48] H.S. Cerqueira, P. Magnoux, D. Martin, M. Guisnet, Appl. Catal. A: Gen. 208 (2001).
TEOM studies on the adsorption of p-xylene in coked FCC catalysts: observation of coke promoting chemical reaction Chi Keng Lee, Lynn F. Gladden, Patrick J. Barrie* Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB23RA, UK Received in revised form 5 July 2004; accepted 6 July 2004 Available online 7 August 2004
Abstract A tapered element oscillating microbalance (TEOM) was used to measure the rates of adsorption and desorption of p-xylene from FCC catalyst at 473 K. The original aim of the work was to investigate whether coke in the catalyst, formed by an earlier reaction with isopropanol at 773 K, influenced the adsorption kinetics. It was actually found that p-xylene reacted in the coked catalyst, even though no reaction occurred in the fresh catalyst at this temperature. This is a rare direct observation of coke causing chemical reaction in a system in which no reaction occurs in the absence of coke. # 2004 Elsevier B.V. All rights reserved. Keywords: Adsorption; Coke; FCC catalyst; Microbalance; TEOM
1. Introduction The fluid catalytic cracking (FCC) process on an oil refinery is a key unit operation in which heavy hydrocarbon molecules are cracked to form smaller more valuable products. The FCC catalyst particles employed contain zeolite crystallites, most commonly rare-earth exchanged zeolite Y, embedded in a matrix that contains alumina, clay and sometimes silica. The zeolite component is responsible for the high activity and selectivity of FCC catalysts. The matrix material has some catalytic activity (e.g. in cracking molecules too large to enter the zeolite) and it provides attrition resistance. It also acts as a heat sink and can reduce the extent of heavy-metal poisoning of the zeolite component [1–3]. There have been a large number of studies on the catalytic performance of zeolites for cracking reactions. For instance, these have explored the influence of pore structure and Si/Al ratio, characterized the different types of acid site that may be present, and proposed reaction mechanisms [4–8]. One particularly important topic that has been studied is the * Corresponding author. Tel.: +44 1223 331864; fax: +44 1223 334796. E-mail address: [email protected] (P.J. Barrie). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.010
effect of coke on the activity and selectivity of zeolite-based catalysts [8–13]. Coke can influence catalytic activity through both site poisoning and pore blocking mechanisms, and it is important to distinguish between the two. For instance, this can be done by measuring the diffusion coefficients of adsorbates within coked zeolites [14,15]. It is also important to know whether the coke is distributed homogeneously throughout the zeolite particles. For instance, coke may preferentially form on the external surface of zeolites crystallites, or will only be produced at the outer edge of the zeolite crystallites if the reaction is diffusion limited [16,17]. Molecular transport within zeolites has been extensively studied because it can influence overall reaction rates, and also because of the use of zeolites in separations processes [18,19]. In a recent paper, we described experiments that used a tapered element oscillating microbalance (TEOM) to measure the rates of adsorption and desorption of n-hexane, n-heptane, n-octane, toluene and p-xylene from catalysts under non-reacting conditions [20]. The catalysts studied were a pure rare-earth exchanged zeolite Y and a commercial FCC catalyst. It was found that the rates of adsorption and desorption were the same for the FCC catalyst as for the pure zeolite Y sample, indicating that mass transport in the
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matrix component of the FCC catalyst is rapid and not a limiting step in the kinetics of adsorption for the hydrocarbons studied. Detailed modelling and numerical simulation of the results was performed and these indicate that the limiting step governing the adsorption kinetics was local transport of molecules at the zeolite–matrix or zeolite– vapour interface [21]. Previous work measuring the adsorption rate of methanol in zeolite NaX found that the apparent surface resistance to adsorption changed during successive measurements; this was attributed to a gradual build-up of coke on the crystal surface which then affected the adsorption kinetics [22,23]. We therefore decided to investigate whether the presence of coke influenced the adsorption and desorption kinetics of pxylene from the FCC catalyst which we had previously studied in detail. As it turned out, we discovered an unusual instance of coke promoting a chemical reaction that would not occur in its absence. While it is known that coke on metal surfaces can act in a beneficial way, this is usually because it enhances product selectivity rather than catalyst activity [24,25]. There are, however, some examples where adsorption and chemical reaction are believed to proceed on the carbonaceous overlayer rather than on the metal itself [26,27]. In the case of reactions in zeolites, several groups of workers have inferred that coke plays an active part in the reaction [28–32]. However, it is rare to observe directly a chemical reaction with coke, and the current work is an example of this phenomenon in a system in which no reaction occurs if coke is absent. This paper first describes the coking of FCC catalyst by reaction with isopropanol vapour at 773 K. It then presents results on the adsorption and desorption of p-xylene vapour in both coked and uncoked catalyst at 473 K. Some experiments were also performed where the base quinoline was introduced into the catalyst prior to the experiments measuring p-xylene adsorption. Mass changes in the catalyst were monitored continuously using a TEOM, while a mass spectrometer was used to provide independent evidence of chemical reaction.
changes can be found by measuring the natural resonance frequency of a tapered quartz element containing the sample of interest [33]. It allows accurate mass measurements to be made rapidly at elevated temperatures under conditions in which the adsorbate vapour passes through the catalyst bed. The flow conditions used, and the fact that the amount of catalyst used is small (typically 60 mg), means that external mass and heat transfer limitations are far less significant than when using other microbalance techniques. Further, it is possible to measure mass in a TEOM every 0.1 s and so rapid changes can be investigated [20]. The composition of the inlet stream to the TEOM could be monitored using a gas chromatograph (GC) equipped with a flame ionization detector, while the composition of the outlet stream of the TEOM could be analysed using either GC or a time-of-flight mass spectrometer (MS). A schematic diagram of the experimental configuration is shown in Fig. 1. All gas flows in the apparatus were controlled by automatic mass flow controllers. The helium flowrates in all experiments were 200 ml/min at STP conditions. The function of the helium purge stream was to ensure that any hydrocarbons desorbing from the catalyst were removed from the balance. All pipes and valves were heated to ensure that no hydrocarbon condensation took place within them. During activation, the catalyst sample is exposed only to helium pretreat gas. However, switching valve 1 exposes the catalyst to the vapour of whichever hydrocarbon is present in the saturator (variously isopropanol, p-xylene and quinoline in this work). It is important that the needle valve on the vent outlet from valve 1 is adjusted to ensure that there is no pressure imbalance in the system when valve 1 is switched. Any pressure imbalance would cause misleading mass measurements as the TEOM is sensitive enough to detect changes in the density of gas within the element.
2. Experimental The commercial FCC catalyst was supplied by BP Oil. It contains rare-earth exchanged zeolite Y in a matrix, and consists of spherical particles of approximately 70 mm diameter. The catalyst was steamed at 1089 K for 5 h in order to cause ‘‘ultrastabilisation’’. This process causes dealumination of the zeolite component, but enhances the overall stability of the catalyst [1,2]. It was performed in order to give a catalyst with a composition and structure comparable to that found in FCC units, as the steaming mimics the actual conditions experienced in an FCC unit. The FCC catalyst was placed at the bottom of the quartz element of a Rupprecht and Patashnick PMA 1500 TEOM. The TEOM is a form of inertial balance in which mass
Fig. 1. Diagram of the experimental configuration used.
C.K. Lee et al. / Applied Catalysis A: General 274 (2004) 269–274
Fig. 2. Summary of the different treatments experienced by the sample during the experiments. Sample mass was measured by the TEOM during each of the four experiments labelled 1–4.
A summary of the different experiments performed is shown in Fig. 2. The element resonance frequency, and thus sample mass, was monitored at all stages of this process, using a time resolution of 0.3 s per data point. For comparison purposes, some experiments were performed using inert quartz particles in the microbalance rather than FCC catalyst.
3. Results and discussion The main aim of reacting the FCC catalyst with isopropanol in this work was the formation of significant amounts of coke. Isopropanol is known to undergo dehydration to
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form propene within acidic zeolites [34,35]. Propene readily reacts further at elevated temperatures to produce a range of aliphatic hydrocarbons [36,37], some of which undergo cyclisation reactions leading to the formation of aromatic and polyaromatic species that ultimately become known as coke [38,39]. While it has been suggested that the initial stage of the propene reaction is direct protonation at a Brønsted acid site to give a carbenium ion [40], it is now believed that adsorbed propene actually forms an alkoxide species which undergoes rapid reaction with other hydrocarbon molecules at elevated temperatures [41–43]. Indeed carbenium ions are now more commonly treated as the highenergy transition states in the reaction mechanism rather than as intermediates [44]. Fig. 3 shows the mass changes occurring during the exposure of the FCC catalyst to isopropanol vapour for 1 h. The initial increase in mass at time t = 400 s is due to adsorption of isopropanol which reacts rapidly to give light hydrocarbon products. The gradual increase in mass over the next hour reflects the formation of heavy products within the catalyst. At time t = 4000 s, desorption is allowed to occur. Even after long desorption periods at 773 K, significant amounts of coke are retained within the catalyst; in the case of the results in Fig. 3, the coke content of the catalyst is 0.4 wt.%. Fig. 4 shows the mass spectrum obtained during the isopropanol experiment with FCC catalyst. For comparison purposes, the mass spectrum recorded with quartz particles in the microbalance at 773 K is also shown. When no catalyst is present, there is no reaction and the mass spectrum obtained corresponds well to that of pure isopropanol [45]. The weak peak at 59 mass units corresponds to M+–H, and the strong peak at 45 mass units corresponds to M+– CH3. With the FCC catalyst, no isopropanol is detected in the mass spectrum, indicating 100% conversion has been
Fig. 3. Mass measurements during the treatment of FCC catalyst with isopropanol at 773 K. At time t = 0 s, the catalyst is in a pure helium environment. Valve 1 is switched at time t = 400 s so that isopropanol (partial pressure 0.47 bar) flows through the catalyst bed. At time t = 4000 s, valve 1 is switched back again, and so desorption takes place in pure helium after this time.
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Fig. 4. Mass spectra obtained on the products when isopropanol is passed through (a) quartz particles, (b) FCC catalyst, at 773 K.
achieved, while the expected major products are present. Propene is responsible for the cluster of peaks between 37 and 42 and also for the peak at 27 mass units, while water gives the peak at 18 mass units [45]. There are also weak peaks at 55, 69 and 84 mass units. These may be assigned to hexene isomers and indicate that oligomerisation reactions are taking place. The catalyst that had been coked by the isopropanol reaction was then exposed to a 60 s pulse of p-xylene vapour at 473 K. The TEOM results are shown in Fig. 5, together with results from the same experiment on an uncoked FCC catalyst. It had been anticipated that the presence of coke might affect the kinetics of adsorption and desorption (e.g. by pore blocking or by affecting the surface barrier to transport at the particle surface). Instead, it is apparent that the rate of adsorption is the same in the coked and fresh catalyst. Surprisingly, though, there is a gradual increase in mass of the coked catalyst during the p-xylene pulse.
Further, even after long desorption times, the mass of the coked FCC catalyst does not return to its value before the pxylene pulse. These results imply that p-xylene must have reacted in the coked FCC catalyst sample and formed further coke within it. This is an interesting result, as no reaction occurred using the fresh catalyst under identical conditions. Fig. 6 shows mass spectra recorded during the p-xylene pulses on both the fresh and coked FCC catalyst samples. The mass spectrum with the fresh FCC catalyst corresponds to that expected for pure p-xylene [45]. The major peaks are at 106 and 91 mass units, with a weaker peak at 105 mass units. These correspond to M+, M+–CH3 and M+–H species, respectively. The mass spectrum obtained with the coked FCC catalyst is quite different. As well as peaks from unreacted p-xylene, it contains additional peaks at 133 and 148, a trace peak at 175, and a change in the relative intensity of the peaks at 105 and 106 mass units. This confirms that p-xylene has reacted with the coked catalyst, even though no reaction occurs in the absence of coke. The most likely product that is consistent with the mass spectrum is dimethylisopropylbenzene formed from alkylation of the p-xylene molecule. This would explain the peaks observed at 148 (M+), 133 (M+–CH3) and 105 (M+–CH3– C2H4) [45,46]. One can go further and speculate that a second alkylation step might occur to give dimethyldiisopropylbenzene as another product. This would have its strongest peak at 175 mass units, and a weak peak is observed at this position in the experimental mass spectrum. It is interesting that the apparent product of the reaction between the coke and p-xylene at 473 K comes from the addition of C3 species given that the coke was originally formed from reactive C3 species. It is very unlikely that there is any residual isopropanol present in the catalyst. Mass spectra showed that 100% conversion of isopropanol was achieved, and a period of 1 h in helium at 773 K should be more than adequate to ensure complete removal of isopropanol even if it had not all reacted. While it might be
Fig. 5. Mass measurements at 473 K for a 60 s pulse of p-xylene (partial pressure 0.058 bar) on fresh FCC catalyst and on FCC catalyst containing coke from the isopropanol reaction. For the sake of clarity, not all data points recorded by the TEOM are shown.
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Fig. 6. Mass spectra obtained during the p-xylene pulses at 473 K using (a) fresh FCC catalyst, (b) coked FCC catalyst.
envisaged that some isopropanol molecules could be trapped in the catalyst by steric blocking due to the presence of coke, such molecules would not themselves be accessible to a larger molecule such as p-xylene. The possibility of the chemical reaction being between residual isopropanol and pxylene can thus be excluded. It is also highly unlikely that C3 carbenium ions can be present in the sample given that they have been found not to be stable species in zeolite Y even at low temperatures [41–43]. The reaction at 473 K must therefore be between pxylene and the coke species formed during the isopropanol reaction. The coke present will be ‘‘high temperature’’ coke as it was formed at 773 K. Structural characterization of coke formed from propene and other reactants in steamed zeolite Y at high temperature have established that it consists mainly of polyaromatic molecules that contain four to six aromatic rings, and that these rings are likely to have alkyl
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substituents [47,48]. If the coke molecules have isopropyl substituents, which is plausible as they were formed from isopropanol and propene reactions, then a transalkylation reaction between p-xylene and coke might occur in a similar fashion to that envisaged during the liquid-phase isopropylation of naphthalene [28,29]. Such a reaction would still require the presence of acid sites. In order to test this hypothesis, additional experiments were performed in which quinoline was adsorbed onto the catalyst prior to exposure with p-xylene. It was found that no reaction occurs with pxylene at 473 K using a coked catalyst that had been exposed to quinoline. Thus the alkylation reaction with coke does indeed require acid sites to be present within the catalyst. Experiments were also performed using a series of pxylene pulses on the same coked FCC catalyst. The TEOM results for a series of six pulses, each of 1 min duration, are shown in Fig. 7. These show continued reaction between pxylene and coke during each pulse, with an increasing amount of residual carbonaceous material remaining in the catalyst after each pulse. The increase in mass detected, after allowing for desorption of p-xylene and products after each pulse is 0.024, 0.035, 0.047, 0.058, 0.065 and 0.068%, respectively. This small increase in mass does not necessarily reflect condensation reactions on the polyaromatic species already present, but may be due to the formation of ‘‘low temperature’’ coke [47] by the proposed alkylation reaction. Low temperature coke consists of molecules that are strongly adsorbed and not very volatile. For instance, it is unlikely that products resulting from more than one alkylation of a p-xylene molecule will be able to desorb completely from the catalyst at a temperature of only 473 K. Finally, it is worth commenting on whether coke affects the rates of adsorption and desorption of p-xylene at 473 K given that this was the original aim of the work. The rates of adsorption and desorption of p-xylene at 473 K on the catalyst that had been coked and then treated with quinoline
Fig. 7. Mass measurements at 473 K for a series of 60 s pulses of p-xylene on FCC catalyst containing coke from the isopropanol reaction.
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(so no reaction occurred) were the same as those measured on the fresh FCC catalyst. This observation shows that the limiting transport step affecting the adsorption kinetics of pxylene at 473 K is not affected by the presence of coke or quinoline at the loadings studied. This is in line with our previous conclusion that local transport at the zeolite–matrix interface is the limiting transport step [21].
4. Conclusions Coke formed by the reaction of isopropanol at 773 K in FCC catalyst is not an inert chemical species. Instead, it causes an alkylation reaction of p-xylene at 473 K to occur. There is no chemical reaction at this temperature in the absence of coke. The reaction in the coked catalyst requires the presence of acid sites to proceed as it was found that adsorption of the base quinoline caused loss of activity of the coked catalyst.
Acknowledgements We thank BP Oil and the Department of Trade and Industry, UK, for financial support of this work. We also thank Dr. Graham Ketley and Dr. Michael Hodges (BP Oil) for their help with this project.
References [1] P.B. Venuto, E.T. Habib Jr., Fluid Catalytic Cracking with Zeolite Catalysts, Marcel Dekker, New York, 1979. [2] C.N. Satterfield, Heterogeneous Catalysis in Practice, McGraw-Hill, New York, 1980. [3] J. Scherzer, Appl. Catal. 75 (1991) 1. [4] A. Corma, Catal. Lett. 22 (1993) 33. [5] J.A. Rabo, G.J. Gajda, Catal. Rev. Sci. Eng. 31 (1989) 385. [6] W.E. Farneth, R.J. Gorte, Chem. Rev. 95 (1995) 615. [7] A. Corma, A.V. Orchille´ s, Micropor. Mesopor. Mater. 35 (2000) 21. [8] K.A. Cumming, B.W. Wojciechowski, Catal. Rev. Sci. Eng. 38 (1996) 101. [9] J.B. Butt, E.E. Petersen, Activation, Deactivation and Poisoning of Catalysts, Academic Press, London, 1988. [10] S. Bhatia, J. Beltramini, D.D. Do, Catal. Rev. Sci. Eng. 31 (1989) 431. [11] H.G. Karge, Stud. Surf. Sci. Catal. 58 (1991) 531. [12] M. Guisnet, P. Magnoux, Appl. Catal. 54 (1989) 1. [13] H.C. Beirnaert, J.R. Alleman, G.B. Marin, Ind. Eng. Chem. Res. 40 (2001) 1337. [14] B. Paweewan, P.J. Barrie, L.F. Gladden, Appl. Catal. A: Gen. 167 (1998) 353.
[15] B. Paweewan, P.J. Barrie, L.F. Gladden, Appl. Catal. A: Gen. 185 (1999) 259. [16] L.G. Okkel, V.B. Fenelonov, V.N. Romannikov, L.P. Soloveva, Kinet. Catal. 33 (1992) 140. [17] J.-L. Bonardet, T. Domeniconi, P. N’Gokoli-Kekele, M.-A. SpringuelHuet, J. Fraissard, Langmuir 15 (1999) 5836. [18] J. Ka¨ rger, D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992. [19] N.Y. Chen, T.F. Degnan Jr., C.M. Smith, Molecular Transport and Reaction in Zeolites: Design and Application of Shape Selective Catalysts, VCH Publishers, New York, 1994. [20] C.K. Lee, S. Ashtekar, L.F. Gladden, P.J. Barrie, Chem. Eng. Sci. 59 (2004) 1131. [21] P.J. Barrie, C.K. Lee, L.F. Gladden, Chem. Eng. Sci. 59 (2004) 1139. [22] P. Grenier, F. Meunier, P.G. Gray, J. Ka¨ rger, Z. Xu, D.M. Ruthven, Zeolites 14 (1994) 242. [23] P. Grenier, V. Bourdin, L.M. Sun, F. Meunier, AIChE J. 41 (1995) 2047. [24] P.G. Menon, J. Mol. Catal. 59 (1990) 207. [25] G.C. Bond, Appl. Catal. A: Gen. 149 (1997) 3. [26] G.E. Vrieland, P.G. Menon, Appl. Catal. 77 (1991) 1. [27] A.A. Barresi, M. Cittadini, A. Zucca, Appl. Catal. B: Environ. 43 (2003) 27. [28] M. Guisnet, J. Mol. Catal. A: Chem. 182–183 (2002) 367. [29] G. Colon, I. Ferino, E. Rombi, P. Magnoux, M. Guisnet, React. Kinet. Catal. Lett. 63 (1998) 3. [30] Z. Da, P. Magnoux, M. Guisnet, Catal. Lett. 61 (1999) 203. [31] P. Andy, N.S. Gnep, M. Guisnet, E. Benazzi, C. Travers, J. Catal. 173 (1998) 322. [32] B. Arstad, B.S. Kolboe, J. Am. Chem. Soc. 123 (2001) 8137. [33] H. Patashnick, E.G. Rupprecht, Res. Dev. 28 (1986) 74. [34] E.A. Lombardo, J.M. Dereppe, G. Marcelin, W.K. Hall, J. Catal. 114 (1988) 167. [35] C.P. Bezoukhanova, Y.A. Kalvachev, Catal. Rev. Sci. Eng. 36 (1994) 125. [36] A.K. Ghosh, R.A. Kydd, J. Catal. 100 (1986) 185. [37] F. Geobaldo, G. Spoto, S. Bordiga, C. Lamberti, A. Zecchina, J. Chem. Soc., Faraday Trans. 93 (1997) 1243. [38] P. Magnoux, F. Machado, M. Guisnet, W.C. Conner, H. Schulz, N.A. Prokudina, S. Csicsery, P.G. Menon, J.P. Fraissard, Stud. Surf. Sci. Catal. 75 (1993) 435. [39] K. Moljord, P. Magnoux, M. Guisnet, Appl. Catal. A: Gen. 122 (1995) 21. [40] M.C. Grady, R.J. Gorte, J. Phys. Chem. 89 (1985) 1305. [41] J.F. Haw, B.R. Richardson, I.S. Oshiro, N.D. Lazo, J.A. Speed, J. Am. Chem. Soc. 111 (2052) 1989. [42] J.F. Haw, T. Xu, Adv. Catal. 42 (1998) 115. [43] A. Bhan, Y.V. Joshi, W.N. Delgass, K.T. Thomson, J. Phys. Chem. B 107 (2003) 10476. [44] V.B. Kazansky, Catal. Today 51 (1999) 419. [45] NIST Chemistry WebBook. http://webbook.nist.gov/. [46] D.H. Williams, I. Fleming, Spectroscopic Methods in Organic Chemistry, McGraw-Hill, London, 1995. [47] M. Guisnet, P. Magnoux, Appl. Catal. A: Gen. 212 (2001) 83. [48] H.S. Cerqueira, P. Magnoux, D. Martin, M. Guisnet, Appl. Catal. A: Gen. 208 (2001).