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n-butene alkylation
Applied Catalysis A: General 281 (2005) 215–223 www.elsevier.com/locate/apcata
Hydrogenative regeneration of a Pt/La-Y zeolite catalyst deactivated in the isobutane/n-butene alkylation Raoul Klingmann, Rouven Josl, Yvonne Traa, Roger Gla¨ser, Jens Weitkamp* Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany Received 8 September 2004; received in revised form 23 November 2004; accepted 24 November 2004 Available online 18 January 2005
Abstract The formation of carbonaceous deposits during the alkylation of isobutane with 1-butene was investigated on a La-Y zeolite catalyst loaded with 0.4 wt.% of platinum in a continuous-flow stirred tank reactor at 75 8C in the liquid phase. By combined elemental analysis and UV–vis spectroscopy, the amount and the nature of the coke deposits were found to change significantly with time-on-stream. Olefins formed by cracking of isododecyl- or higher carbocations are supposed to be important precursors of the carbonaceous deposits, as they form unsaturated carbocations blocking the acid sites. The hydrogenative regeneration of coked alkylation catalysts was studied in the gas phase. Under suitable regeneration conditions, e.g. at a hydrogen pressure of 15 bar and a temperature of 300 8C, the alkylation activity can be fully restored. # 2004 Elsevier B.V. All rights reserved. Keywords: Alkylation; Isobutane/butene alkylation; Deactivation; Hydrogenative regeneration; Coke characterization; Hydrocracking; UV–vis spectroscopy; Alkenyl cations
1. Introduction Alkylate produced from isobutane and butenes is a most valuable gasoline blending component [1]. In 2003, about 77 106 t of alkylate were produced worldwide [2]. Currently, sulfuric acid or hydrofluoric acid are used as catalysts for isobutane/butene alkylation. These liquid catalysts suffer from a number of disadvantages: hydrofluoric acid is highly toxic, and the acid consumption in processes using sulfuric acid is considerable. It is hence desirable to develop novel alkylation catalysts on the basis of environmentally benign solids [3,4]. Zeolite catalysts are good candidates for this application, but they deactivate after a relatively short time-on-stream. To cope with this deactivation, a periodical or continuous regeneration is needed. Garwood et al. [5] and Kirsch et al. [6] were the first to explore the potential of zeolites as catalysts for the * Corresponding author. Tel.: +49 711 685 4060; fax: +49 711 685 4065. E-mail address: [email protected] (J. Weitkamp). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.11.032
isobutane/alkene alkylation. Since then, a broad variety of zeolitic catalysts were tested [7] among which faujasite- and Beta-type zeolites predominate. It appears that, in the most recent studies, there is trend towards faujasite-based alkylation catalysts [8]. The time-on-stream behavior of solid alkylation catalysts can be divided into three stages [7,9,10]: (i) the alkylation stage at the onset of the reaction is characterized by a complete butene conversion and a high selectivity to the desired trimethylpentanes; (ii) in the subsequent stage of catalyst deactivation, the butene conversion drops rapidly, and the product spectrum gradually shifts from highly branched isoalkanes to moderately branched isoalkenes; (iii) in the final oligomerization stage, the butene conversion is essentially constant, but the catalyst has completely lost its capability to convert butenes with isobutane into alkylate, rather it produces oligomers from the butenes. While it is clear that these drastic changes in the catalyst behavior have their origin in the build-up of carbonaceous deposits, the precise mechanism of their formation and location continues to be a matter of debate: Both poisoning of the active sites by
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irreversible adsorption of product molecules and obstruction of the active sites by pore blocking or pore filling have been envisaged [11]. To characterize the deposits responsible for the deactivation of zeolite catalysts in the isobutane/butene alkylation, various spectroscopic and analytical methods have been applied. Among these are 13C NMR spectroscopy [12,13], IR spectroscopy [14], UV–vis spectroscopy [15], temperature-programmed oxidation [16], matrix assisted laser desorption ionization-time of flight spectroscopy (MAL DI-TOF) [14] and dissolution of the zeolite in HF with subsequent extraction and analysis of the organic phase [17,18]. However, except for the investigations of Pater et al. [19] and Feller et al. [14], all these prior studies were undertaken with catalysts that had already passed the alkylation stage. Pater et al. investigated the isobutane/n-butene alkylation in a fixed-bed plug flow reactor (PFR), which is now widely considered to be inadequate for this reaction [20] because of the concentration gradients along the catalyst bed. As shown by Querini and Roa [16], coke precursors can be desorbed from the upper part of the catalyst bed and readsorbed in the lower part where consecutive transformations can occur. Feller et al. characterized a La-X zeolite catalyst by MALDI-TOF after having stopped the reaction during the alkylation stage [14]. However, with this method, it is impossible to study the carbonaceous deposits while they are still located on the catalyst. Due to the rapid deactivation by coke deposition, any industrial isobutane/alkene alkylation process applying solid acid catalysts requires a periodical or continuous catalyst regeneration. To remove the carbonaceous deposits from the zeolite, various options are available, viz. extraction with a liquid or a supercritical fluid [21], combustion, e.g. in air [16] or hydrocracking [22]. While regeneration by hydrocracking has been proposed as early as 1975 [22] and thereafter mentioned in several patents [23,24], it has hardly been dealt with in the open literature. Panattoni and Querini found that a complete regeneration of a platinum-containing zeolite La-Y could not be achieved by treatment with hydrogen at temperatures up to 700 8C and a hydrogen pressure of 1 bar [25]. In the present study, isobutane/1-butene alkylation was performed in the liquid phase using a continuous-flow stirred tank reactor and a Pt/La-Y zeolite catalyst and interrupted at different times-on-stream during the alkylation stage and during the stage of rapid catalyst deactivation. The deactivated catalysts were characterized by UV–vis spectroscopy, nitrogen adsorption and chemical analysis. From the combined results information about the deactivation mechanism was gathered. Moreover, regeneration of the catalysts by hydrocracking was studied at various temperatures up to 300 8C and elevated hydrogen pressure of 15 bar. The products observed during hydrocracking of the carbonaceous deposits are compared with those of hydrocracking of a long-chain alkane, viz. n-dodecane, as a model
compound [26]. From this comparison, conclusions concerning the mechanism of regeneration will be deduced.
2. Experimental 2.1. Catalyst preparation and characterization The catalyst was prepared by ion exchange of a commercial zeolite Na-Y (Strem Chemicals Co., Lot.Nr.148960) with an aqueous solution of La(NO3)3 (0.2 mol dm 1, Merck) at 80 8C for 2 h. This ion exchange was repeated once. After filtration, washing with deionized water and drying under ambient conditions, the zeolite was heated in flowing air (50 cm3 min 1) with a rate of 3 8C min 1 up to a final temperature of 400 8C and kept at this temperature for 1 h, before it was allowed to cool down to room temperature. The obtained zeolite was hydrated over a saturated aqueous solution of Ca(NO3)2 until its mass became constant. Two consecutive ion exchange steps with an aqueous solution of NH4NO3 (0.5 mol dm 1, Fluka) at 80 8C for 2 h were applied, followed by another ion exchange with aqueous La(NO3)3 solution as described above. To an aqueous slurry of the obtained zeolite, a solution of Pt(NH3)4Cl2 (Fluka) was added dropwise under vigorous stirring at room temperature. The amount of platinum was 0.4 wt.% (based on the anhydrous zeolite). The elemental composition of the catalyst was determined by ICP-AES (Perkin Elmer Plasma 400) after dissolution in an aqueous solution of HF (10 wt.%). The concentrations of Lewis and Brønsted acid sites were calculated from FT-IR spectroscopy after pyridine adsorption at 200 8C, as described earlier [27], using the extinction coefficients determined by Emeis [28]. Table 1 shows the main physicochemical properties of the catalyst. 2.2. Catalytic experiments and catalyst regeneration The zeolite powder was pressed without a binder and used with a particle size between 0.40 and 0.63 mm. The catalyst was activated in a fixed-bed reactor. To achieve a high dispersion of the noble metal, it was first heated in flowing air (50 cm3 min 1) with a rate of 2 8C min 1 to a final temperature of 300 8C. After 16 h, the gas flow was switched to nitrogen (50 cm3 min 1) for 10 min, then to hydrogen (50 cm3 min 1) for 4 h. After cooling down to Table 1 Physicochemical properties of the 0.4Pt/La-Y zeolite catalyst nSi/nAl 3nLa/nAl mNa/mCatalyst mPt/mCatalyst cBrønsted acid sites (mmol/g) cLewis acid sites (mmol/g) Micropore volume (cm3/g)
2.82 1.05 2.4 10 4.0 10 0.33 0.04 0.263
3 3
R. Klingmann et al. / Applied Catalysis A: General 281 (2005) 215–223 Table 2 Reaction conditions employed during the catalytic alkylation experiments mCatalyst (g) pReaction (bar) TReaction (8C) t = VLiquid phase, Reactor/V˙ Feed (h) WHSV1-Butene (h 1)
1.0 20 75 4 0.5
135 8C, the catalyst was quickly transferred to the alkylation reactor, where it was kept under flowing nitrogen (20 cm3 min 1) at 135 8C for 16 h. The alkylation reactions were carried out in a continuous-flow stirred tank reactor (Microclave, Autoclave Engineers Co., V = 50 cm3) equipped with a fixed annular catalyst basket. After cooling to the reaction temperature of 75 8C, the reactor was first filled with 20 g of liquid isobutane by means of a mass flow controller (Brooks Flomega 5881) under nitrogen pressure (ptotal = 20 bar). Then, the premixed isobutane/1-butene feed (nIsobutane/n1-Butene = 9) was continuously fed to the reactor using the same mass flow controller. Isobutane (99.5 vol.%) and 1-butene (99.5 vol.%) were obtained from Air Liquide GmbH and Linde AG, respectively. The main reaction parameters are summarized in Table 2. The reactor effluent was continuously vaporized by means of a heated needle valve and addition of a nitrogen gas flow (100 cm3 min 1) that was preheated to a temperature of 200 8C. An HP-6890 gas chromatograph equipped with on-line sampling, a flame ionization detector and a capillary column (Supelco Petrocol, l = 100 m, di = 0.25 mm, dfilm = 0.5 mm, stationary phase: polydimethylsiloxane, carrier gas: hydrogen (1 cm3 min 1)) were employed for product analysis. After an alkylation experiment, the reactor was depressurized at reaction temperature. Then it was purged with nitrogen (20 cm3 min 1) for ca. 30 min and allowed to cool down to room temperature. To ensure that the properties of the coked catalyst did not change by exposure to air or moisture, the catalyst was stored under nitrogen before placing it in the regeneration reactor. Regeneration of the catalyst was carried out in a hydrogen flow of 40 cm3 min 1 at a pressure of 15 bar in a high-pressure flow-type apparatus with a fixed-bed reactor as described earlier [29]. Heating rates of 100 8C/h up to the final regeneration temperature (200, 250 or 300 8C) were applied. The final temperature was held for a total regeneration time of 7 h 45 min.
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sum of the weight losses due to the desorption of water and the removal of carbonaceous deposits, was determined by TGA up to 950 8C using a heating rate of 20 8C min 1 (Setaram TG DTA 92). The relative amount of oxygen on the catalyst was obtained by subtracting the relative amount of hydrogen and carbon from the total weight loss. As this amount of oxygen was present on the catalyst in the form of water, the relative amount of hydrogen could be divided into one part bound to oxygen (water) and a second part bound to carbonaceous deposits. Micropore volumes were calculated from adsorption isotherms of nitrogen at 196 8C (Micromeritics ASAP 2010). The samples were pretreated at T = 80 8C and p < 10 8 bar overnight. For the UV–vis measurements, the catalyst pellets were powdered in a mortar and placed in a quartz cell. The spectra were recorded in the reflection mode in the 190–700 nm range with a Perkin Elmer Lambda 16 spectrometer at room temperature. The scanning speed was 480 nm min 1. Before recording the spectra, the catalysts were stored in an argon atmosphere to avoid exposure of the catalyst to air or moisture.
3. Results and discussion 3.1. Deactivation of the catalyst Fig. 1 shows the conversion of n-butene and the composition of the reaction products in dependence of time-on-stream. The double-bond shift of 1-butene into cisand trans-2-butene is not considered in the calculation of the conversion, because this isomerization is fast. Under the conditions employed, complete n-butene conversion occurred for ca. 2 h. The desired isooctanes represent the major product fraction which initially accounts for more than 80 wt.%; their yield decreases monotonously with time-on-stream. The isomer distribution of the C8-fraction is shown in Fig. 2. The main components are 2,3,3-, 2,3,4- and
2.3. Characterization of the coked and regenerated catalyst samples The amount of carbon and hydrogen on the coked catalyst samples was determined using elemental analysis (Elementar Vario EL). The carbon contents are given in wt.% of the fully hydrated sample. The nH/nC ratio was calculated by a method combining thermogravimetric analysis (TGA) and elemental analysis: first, the relative amounts of carbon and hydrogen on the catalyst were determined by elemental analysis. Then the total weight loss of the catalyst, i.e. the
Fig. 1. Conversion and product composition during isobutane/1-butene alkylation on 0.4Pt/La-Y.
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Fig. 2. Composition of the isooctane fraction during isobutane/1-butene alkylation on 0.4Pt/La-Y.
2,2,4-TM-Pn, which initially account for more than 85 wt.% of the C8-fraction. Compared to these three trimethylpentane isomers, the primary product one might expect according to the cationic alkylation mechanism, i.e. 2,2,3-TM-Pn, is formed in minor amounts. The nTM-Pn/nDM-Hx ratio initially amounts to 12.2 and drops slowly as the reaction proceeds. Monobranched octane isomers are formed in negligible amounts only. As soon as the conversion begins to drop, i.e. after ca. 2 h, the selectivity to dimethylhexanes, especially 3,4-DM-Hx, increases. Almost identical compositions of the isooctane fraction during isobutane/n-butene alkylation at ca. 80 8C were found earlier by Weitkamp [9] and Rørvik et al. [30] on Ce-Y zeolite catalysts and by Kirsch et al. on a RE-Y zeolite catalyst [31]. Light ends (C5–C7) are formed with a mass fraction around 10 wt.% (cf. Fig. 1). The yield of heavy ends (C9+) increases in a pronounced manner as the reaction proceeds. No unsaturated products could be detected within the first 3 h on-stream. In Fig. 3a, the carbon content of the catalyst and the micropore volume after termination of the alkylation
Fig. 3. (a) Carbon content and micropore volume of the 0.4Pt/La-Y zeolite catalyst in dependence of time-on-stream. (b) UV–vis spectra of the 0.4Pt/ La-Y zeolite catalyst after different times-on-stream.
experiments at different times-on-stream are shown. Within the period of complete n-butene conversion, i.e. within the first 2 h, the amount of carbon deposited on the catalysts increases only slightly and, in-line with this result, the micropore volume decreases only slowly within this time. After 3 h on-stream, when the period of rapid deactivation begins (cf. Fig. 1), the amount of carbonaceous deposits increases steeply, and a concomitant loss in micropore volume from ca. 0.24 to 0.15 cm3 g 1 is observed. This is at variance to results of Pater et al., who found that the rate of coke formation on the zeolite catalyst during alkylation in a fixed-bed plug-flow reactor is high at the beginning of the reaction and then slows down significantly [19], though their experiments were performed at much lower space velocity and higher paraffin to olefin ratio. A distinctive correlation between the build-up of coke inside the pores of the catalyst and the conversion or selectivity of the alkylation reaction was not found. However, our results are in principal accordance with those of Feller et al. [14]. Comparing the signal intensity of MALDI-TOF spectra of La-X zeolite catalysts, that had been used in isobutane/2-butene alkylation at 75 8C for different times, the latter authors found a steep increase in the signal intensity, which paralleled the amount of carbonaceous deposits on the catalyst, at a timeon-stream where the conversion started to drop. The nH/nC ratio of the deposits on the 0.4Pt/La-Y catalyst was 1.6 after 6 h time-on-stream. This is significantly lower than the value of 2.0 expected for a product formed by butene oligomerization and essentially in-line with earlier studies. Weitkamp and Maixner found an nH/nC ratio of 1.8 by elemental analysis for a La,Na-Y zeolite catalyst that had already entered the oligomerization stage [12], while Feller et al. calculated a formula based on MALDI-TOF spectra of CnH2n 4 for the coke molecules desorbed from a deactivated La-X catalyst with a maximum in the range between C17 and C21, which corresponds to an nH/nC ratio of ca. 1.7 [14]. Unsaturated carbocations formed during the alkylation stage seem to be responsible for the relatively low nH/nC ratios of the deposits. Indeed, such enylic cations could be observed by UV–vis spectroscopy (Fig. 3b). The absorption bands were assigned after Fo¨ rster et al. [32]. The intensity of the band at 305 nm, originating from monoenyl carbocations, increases during the first 3 h on-stream and then remains nearly constant. The intensity of the band at 370 nm representing dienyl carbocations increases only during the first hour on-stream. A small band at 450 nm assigned to trienyl carbocations could only be observed within the early stage of the reaction. The formation of such unsaturated carbocations can be rationalized in terms of consecutive hydrogen transfer reactions [33,34]. Donation of a hydride ion from an olefin to a saturated carbocation results in an unsaturated carbocation and an alkane. To some extent, the unsaturated carbocations desorb as dienes. Further hydride transfer from the dienes to saturated carbocations leads to the formation of dienylic cations. In this way, the deposits formed in the early stage of the alkylation reaction can act as
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efficient hydride donors [35,36], in a similar manner as the acid-soluble oils formed during isobutane/butene alkylation catalyzed by sulfuric acid [37]. Scho¨ llner et al. first postulated the key role of unsaturated carbocations in the deactivation of zeolite catalysts used in the isobutane/butene alkylation [38]. The resonancestabilized unsaturated carbocations are more stable than the tert-butyl cation or other saturated carbocations [39,40]. Desorption of these enylic cations via hydride transfer with isobutane is therefore unlikely. They remain almost irreversibly adsorbed and poison the acid sites. Tert-butyl cations, that stand at the beginning of a new catalytic alkylation cycle, cannot be restored, i.e. the concentration of sites available for the alkylation decreases. It can be seen from Fig. 3b that the concentration of monoenylic carbocations (l = 305 nm) on the catalyst surface increases during the initial 2 or 3 h, i.e. as long as the catalyst maintains a high alkylation activity (cf. Fig. 1). Once the butene conversion starts to drop, only negligible amounts of further enylic carbocations are formed. It has been shown by Pine et al. [41], Jacquinot et al. [42] and de la Puente and Sedran [43] that the rate of hydride transfer on zeolite Y is strongly dependent on the density of acid sites. As the acid site density gradually diminishes due to poisoning by the unsaturated carbocations, the hydride transfer ability of the catalyst decreases. The enylic carbocations are themselves formed by reactions involving hydride transfer, it can hence be argued that these species are formed only as long as the catalyst possesses a sufficiently high hydride transfer activity. Carbocations with two or more double bonds can undergo cyclization and form naphthenic rings with side chains, which can in turn form aromatics in consecutive hydrogen transfer reactions. This could explain the disappearance of the small absorption band at 450 nm at higher times-onstream. It is noteworthy, however, that Paze` et al. [44] found evidence for the cyclization of polyenylic carbocations adsorbed on an acidic zeolite only at temperatures above 300 8C. In our study, no absorption bands of aromatics could be detected within the alkylation stage. After 5–6 h time-onstream, a weak shoulder in the band of the monoenyl carbocations at approximately 260 nm appears, which could originate from aromatic compounds. This was also inferred by several groups who found that the deposits contain bulky organic compounds. Using MALDI-TOF-MS [45] or characterizing the extracted organic phase by GC–MS, UV–vis or 13C NMR spectroscopy after dissolution of the coked catalysts in aqueous HF [18,46], it was concluded that the deposits contain two naphthenic or aromatic rings. In the cationic alkylation mechanism [7], isododecyl cations can be formed by addition of butene to isooctyl cations. These C12-carbocations can undergo type A rearrangements and b-scissions [7], whereby smaller carbocations and olefins are formed. At the low temperatures applied in isobutane/butene alkylation on zeolites, only type A b-scissions can occur, and they lead to olefins with at least
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Scheme 1. Formation of unsaturated carbocations from cracking products.
one tertiary carbon atom. As an example, type A b-scission of the 2,3,4,5,5-pentamethylheptyl-3 cation is depicted in Scheme 1. The fragments formed in the type A b-scission of this cation are 3,4-dimethyl-2-pentene and the 2-methylbutyl-2 cation. These primary moieties can further react in several ways (cf. Scheme 1): (A) alkylation, i.e. the reverse reaction of the preceding cracking, (B) hydride transfer from the olefin to the carbocation resulting in 2-methylbutane and an allylic carbocation and (C) alkylation of a saturated carbocation (R = alkyl) at a neighboring acid site by the olefin or protonation (R = H) of the olefin by an adjacent free Brønsted acid site. As hydride abstraction from tertiary carbon atoms, especially in allylic position, proceeds much easier than from secondary or primary C-atoms of the feed butene molecules [47], it is hence reasonable to assume that the branched olefins formed in cracking reactions are the main precursors for the unsaturated carbocations formed via route (B). The decrease in the reaction rate of the alkylation due to the loss of active sites by poisoning results in a higher olefin concentration inside the catalyst pores. As a consequence, the oligomerization of olefins is more and more favored over the alkylation reaction. As soon as oligomerization becomes the prevailing reaction, high molecular weight hydrocarbons are formed inside the pores of the catalyst. These oligomers are responsible for the steep increase in the coke content of the catalyst and the concomitant decrease of the micropore volume (Fig. 3a). At this time-on-stream olefins appear in the product, i.e. the carbocations formed during alkylation are no longer desorbed via hydride transfer with isobutane, but by back-donation of a proton to the zeolite. 3.2. Regeneration of the catalyst The coked catalysts were regenerated in the fixed-bed reactor in a hydrogen flow at elevated pressure. Three different temperature programs were applied. It is well known from several related processes, such as skeletal isomerization or reforming, that elevated hydrogen pressure
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Fig. 4. Intensity of the FID signal during regeneration of the 0.4Pt/La-Y zeolite catalyst at different temperatures.
is necessary to suppress the formation of coke deposits and catalyst deactivation [48]. Therefore, a hydrogen pressure of 15 bar was chosen for the regeneration. The total flux of organic compounds released from the catalyst during regeneration was detected with a flame ionization detector (FID). In Fig. 4, the FID signal intensity is plotted versus the time of regeneration for a catalyst that had been used for 3 h in the alkylation. Raising the temperature up to 200 8C results in a maximum signal intensity at 200 8C. Thereafter, the intensity drops slowly within the remaining time of regeneration. If a temperature program up to 300 8C is applied, the signal intensity reaches its maximum at 300 8C, whereupon it drops rapidly to zero. No temperatures above 300 8C were applied in the regeneration of the catalyst, since this would have resulted in a diminution of the concentration of Brønsted acid sites [49]. An as high as possible concentration of Brønsted acid sites is known to be favorable for alkylation catalysts [50]. These results are in principal accordance with Querini’s work [51], who also observed a maximum in FID signal intensity at ca. 300 8C in the regeneration of a 0.5 Pt/La,H-Y zeolite catalyst which had been used in the isobutane/nbutene alkylation. Moreover, our results are in-line with studies on hydrocracking of long-chain alkanes on similar zeolite catalysts. For example, it has been reported that, on a Pt/Ca-Y zeolite catalyst, hydrocracking of n-dodecane starts around 250 8C and is complete at 300 8C [26]. Fig. 5a shows the residual carbon contents of the zeolite catalyst after its regeneration at different temperatures. Applying a final regeneration temperature of 300 8C results in a decrease of the carbon content from ca. 3 wt.% before the regeneration to 0.2 wt.% after the regeneration. Lower final temperatures of 200 or 250 8C result in significant higher residual carbon contents. These results support those obtained by observing the total flux of organic compounds released from the catalyst during regeneration (Fig. 4) and show that a regeneration temperature of 300 8C is favorable for removing the coke deposits.
Fig. 5. Carbon content after regeneration of the 0.4Pt/La-Y zeolite catalyst (a) at different temperatures; (b) after different times-on-stream of the alkylation reaction.
Regenerating catalysts after different times-on-stream in the alkylation reaction does not result in markedly different carbon contents after the regeneration at 300 8C (Fig. 5b). This is a remarkable result, because, as shown in Fig. 3a, the amount of carbon on the catalyst does strongly increase with time-on-stream in the alkylation reaction, i.e. before the regeneration. These pronounced differences in the carbon content of the used zeolite catalyst before and after the regeneration must have to do with the composition of the carbonaceous deposits after different times-on-stream: the unsaturated species formed in the early stage of alkylation, as observed by UV–vis spectroscopy, are presumably more difficult to remove by hydrocracking than the long-chain highly branched saturated carbocations formed by olefin oligomerization in the late stage of the reaction. A reason for this could be that the aforementioned unsaturated species can act as precursors for ‘‘hard coke’’ [33]. As these species are mainly formed in the beginning of the alkylation reaction, when the hydride transfer activity is still high, they will be present in similar concentrations on the catalyst after 1, 3 and 5 h of alkylation. The saturated high molecular weight carbocations, which are mainly responsible for the observed increase in carbon content after extended times-onstream in the alkylation, are removed relatively easily by hydrocracking [52]. Fig. 6 shows the distribution of the products obtained during regeneration of the 0.4Pt/La-Y zeolite catalyst, that had been used in the alkylation reaction for 3 h. This distribution changes markedly with increasing temperature. At 200 8C, large amounts of C8 hydrocarbons are released from the catalyst (not shown in Fig. 6). Presumably, these originate from the desorption of physisorbed isooctanes or the displacement of isooctyl cations from the acid sites. Besides isooctanes, cracked products are observed at 200 8. At 300 8C, the products consist predominantly of hydrocracked products, mainly of saturated short-chain molecules, especially butanes. The product distribution very much resembles that obtained by hydrocracking of n-
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Fig. 6. Comparison of the products formed by hydrocracking during regeneration of the 0.4Pt/La-Y zeolite catalyst at 300 8C with the products formed by hydrocracking of n-dodecane on a Pt/Ca-Y zeolite catalyst [26] at 325 8C.
dodecane on a Pt/Ca-Y zeolite catalyst at a similar temperature (Fig. 6) [26]. This can be explained by the fact that after 3 h of alkylation the major part of the coke deposits consists of long-chain saturated oligomers. It appears reasonable to assume that the cracking of these moieties proceeds in a similar manner as that of long-chain alkanes adsorbed from the gas phase [7]. The differences in the product composition may arise from the following fact: During hydrocracking experiments, long-chain hydrocarbons are continuously fed into the reactor. These feed molecules compete with the cracked product molecules for acid sites and thereby suppress the occurrence of secondary cracking reactions [53]. Therefore, it is to be expected that the yield of secondary cracked products in the regeneration surpasses that obtained in the continuous hydrocracking of n-dodecane. In this way, the formation of increased amounts of butanes during the regeneration experiment (Fig. 6), even though it was performed at a slightly lower temperature than the hydrocracking experiments with n-dodecane, may be accounted for. To demonstrate that the hydrogenative regeneration restores the alkylation activity, a catalyst, which had been used for 3 h in the alkylation and subsequently regenerated at 300 8C, was re-used in an alkylation experiment. Both the catalytic activity and the product selectivity (Figs. 7 and 8) were fully restored by the regeneration. Even in a third alkylation reaction following the second regeneration step, the activity and selectivities were essentially equal to those in the first alkylation reaction. The catalyst was characterized after each alkylation and regeneration step using UV– vis spectroscopy (Fig. 9). The intensity of the signals corresponding to monoenylic carbocations decreases slightly with the number of alkylation steps. The reason could be a small decrease of the concentration of acid sites, since these sites are poisoned by coke deposits which cannot be removed during the regeneration. After each regeneration step, UV–vis signals were absent. This shows that polyaromatic species were not formed in detectable
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Fig. 7. Conversion and product composition during isobutane/1-butene alkylation on the 0.4Pt/La-Y zeolite catalyst within three alkylation/regeneration cycles (at the times indicated by the vertical dotted lines, the catalyst was regenerated at 300 8C).
Fig. 8. Composition of the isooctane fraction during isobutane/1-butene alkylation on 0.4Pt/La-X within three alkylation/regeneration cycles (at the times indicated by the vertical dotted lines, the catalyst was regenerated at 300 8C).
Fig. 9. UV–vis spectra of the 0.4Pt/La-Y zeolite catalyst after alkylation and after regeneration within three alkylation/regeneration cycles.
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amounts. However, it cannot be excluded that a slow buildup of such species would have resulted in a gradual deterioration of the alkylation catalyst, if a larger number of alkylation/regeneration cycles had been carried out.
4. Conclusions The formation of enylic cationic species seems to be a decisive step in the deactivation of zeolitic catalysts during isobutane/butene alkylation. These unsaturated carbocations are presumably formed via hydride transfer from olefins originating from cracking of C12+ or higher carbocations. The enylic carbocations remain adsorbed almost irreversibly and poison the acid sites of the catalyst. Thereby, the density of acid sites available for catalyzing the desired alkylation reaction decreases steadily. The resulting higher olefin concentration leads to a rapid deactivation of the catalyst via oligomerization and subsequent blocking or filling of the zeolite pores. Our regeneration experiments have demonstrated that the catalyst activity can be completely restored by applying a temperature program up to 300 8C at a hydrogen pressure of 15 bar without any loss in selectivity to the desired trimethylpentanes. After three cycles of alkylation/regeneration, no absorption bands of polyaromatic compounds were observed in the UV–vis spectrum. Thus, UV–vis spectroscopy can be used as a powerful tool for characterizing the efficiency of the regeneration of zeolite catalysts coked in the alkylation of isobutane with butenes. Glossary DM-Hx: dimethylhexane FID: flame ionization detector FT-IR: Fourier transform infrared ICP-AES: atomic emission spectroscopy by inductively coupled plasma MALDI–TOF: matrix assisted laser desorption ionization–time of flight MS: mass spectroscopy TM-Pn: trimethylpentane PFR: plug flow reactor
Acknowledgements The authors gratefully acknowledge financial support by Lurgi Oel Gas Chemie and Fonds der Chemischen Industrie. Yvonne Traa thanks the Ministerium fu¨ r Wissenschaft, Forschung und Kunst Baden-Wu¨ rttemberg and the Landesstiftung Baden-Wu¨ rttemberg gGmbH for special funding. Roger Gla¨ ser thanks the Dr. Leni Scho¨ ninger Foundation for a habilitation stipend. References [1] L.F. Albright, Ind. Eng. Chem. Res. 42 (2003) 4283. [2] D. Nakamura, Oil Gas J. 101 (49) (2003) 64, and http://ogj.pennnet. com.
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Hydrogenative regeneration of a Pt/La-Y zeolite catalyst deactivated in the isobutane/n-butene alkylation Raoul Klingmann, Rouven Josl, Yvonne Traa, Roger Gla¨ser, Jens Weitkamp* Institute of Chemical Technology, University of Stuttgart, D-70550 Stuttgart, Germany Received 8 September 2004; received in revised form 23 November 2004; accepted 24 November 2004 Available online 18 January 2005
Abstract The formation of carbonaceous deposits during the alkylation of isobutane with 1-butene was investigated on a La-Y zeolite catalyst loaded with 0.4 wt.% of platinum in a continuous-flow stirred tank reactor at 75 8C in the liquid phase. By combined elemental analysis and UV–vis spectroscopy, the amount and the nature of the coke deposits were found to change significantly with time-on-stream. Olefins formed by cracking of isododecyl- or higher carbocations are supposed to be important precursors of the carbonaceous deposits, as they form unsaturated carbocations blocking the acid sites. The hydrogenative regeneration of coked alkylation catalysts was studied in the gas phase. Under suitable regeneration conditions, e.g. at a hydrogen pressure of 15 bar and a temperature of 300 8C, the alkylation activity can be fully restored. # 2004 Elsevier B.V. All rights reserved. Keywords: Alkylation; Isobutane/butene alkylation; Deactivation; Hydrogenative regeneration; Coke characterization; Hydrocracking; UV–vis spectroscopy; Alkenyl cations
1. Introduction Alkylate produced from isobutane and butenes is a most valuable gasoline blending component [1]. In 2003, about 77 106 t of alkylate were produced worldwide [2]. Currently, sulfuric acid or hydrofluoric acid are used as catalysts for isobutane/butene alkylation. These liquid catalysts suffer from a number of disadvantages: hydrofluoric acid is highly toxic, and the acid consumption in processes using sulfuric acid is considerable. It is hence desirable to develop novel alkylation catalysts on the basis of environmentally benign solids [3,4]. Zeolite catalysts are good candidates for this application, but they deactivate after a relatively short time-on-stream. To cope with this deactivation, a periodical or continuous regeneration is needed. Garwood et al. [5] and Kirsch et al. [6] were the first to explore the potential of zeolites as catalysts for the * Corresponding author. Tel.: +49 711 685 4060; fax: +49 711 685 4065. E-mail address: [email protected] (J. Weitkamp). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.11.032
isobutane/alkene alkylation. Since then, a broad variety of zeolitic catalysts were tested [7] among which faujasite- and Beta-type zeolites predominate. It appears that, in the most recent studies, there is trend towards faujasite-based alkylation catalysts [8]. The time-on-stream behavior of solid alkylation catalysts can be divided into three stages [7,9,10]: (i) the alkylation stage at the onset of the reaction is characterized by a complete butene conversion and a high selectivity to the desired trimethylpentanes; (ii) in the subsequent stage of catalyst deactivation, the butene conversion drops rapidly, and the product spectrum gradually shifts from highly branched isoalkanes to moderately branched isoalkenes; (iii) in the final oligomerization stage, the butene conversion is essentially constant, but the catalyst has completely lost its capability to convert butenes with isobutane into alkylate, rather it produces oligomers from the butenes. While it is clear that these drastic changes in the catalyst behavior have their origin in the build-up of carbonaceous deposits, the precise mechanism of their formation and location continues to be a matter of debate: Both poisoning of the active sites by
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irreversible adsorption of product molecules and obstruction of the active sites by pore blocking or pore filling have been envisaged [11]. To characterize the deposits responsible for the deactivation of zeolite catalysts in the isobutane/butene alkylation, various spectroscopic and analytical methods have been applied. Among these are 13C NMR spectroscopy [12,13], IR spectroscopy [14], UV–vis spectroscopy [15], temperature-programmed oxidation [16], matrix assisted laser desorption ionization-time of flight spectroscopy (MAL DI-TOF) [14] and dissolution of the zeolite in HF with subsequent extraction and analysis of the organic phase [17,18]. However, except for the investigations of Pater et al. [19] and Feller et al. [14], all these prior studies were undertaken with catalysts that had already passed the alkylation stage. Pater et al. investigated the isobutane/n-butene alkylation in a fixed-bed plug flow reactor (PFR), which is now widely considered to be inadequate for this reaction [20] because of the concentration gradients along the catalyst bed. As shown by Querini and Roa [16], coke precursors can be desorbed from the upper part of the catalyst bed and readsorbed in the lower part where consecutive transformations can occur. Feller et al. characterized a La-X zeolite catalyst by MALDI-TOF after having stopped the reaction during the alkylation stage [14]. However, with this method, it is impossible to study the carbonaceous deposits while they are still located on the catalyst. Due to the rapid deactivation by coke deposition, any industrial isobutane/alkene alkylation process applying solid acid catalysts requires a periodical or continuous catalyst regeneration. To remove the carbonaceous deposits from the zeolite, various options are available, viz. extraction with a liquid or a supercritical fluid [21], combustion, e.g. in air [16] or hydrocracking [22]. While regeneration by hydrocracking has been proposed as early as 1975 [22] and thereafter mentioned in several patents [23,24], it has hardly been dealt with in the open literature. Panattoni and Querini found that a complete regeneration of a platinum-containing zeolite La-Y could not be achieved by treatment with hydrogen at temperatures up to 700 8C and a hydrogen pressure of 1 bar [25]. In the present study, isobutane/1-butene alkylation was performed in the liquid phase using a continuous-flow stirred tank reactor and a Pt/La-Y zeolite catalyst and interrupted at different times-on-stream during the alkylation stage and during the stage of rapid catalyst deactivation. The deactivated catalysts were characterized by UV–vis spectroscopy, nitrogen adsorption and chemical analysis. From the combined results information about the deactivation mechanism was gathered. Moreover, regeneration of the catalysts by hydrocracking was studied at various temperatures up to 300 8C and elevated hydrogen pressure of 15 bar. The products observed during hydrocracking of the carbonaceous deposits are compared with those of hydrocracking of a long-chain alkane, viz. n-dodecane, as a model
compound [26]. From this comparison, conclusions concerning the mechanism of regeneration will be deduced.
2. Experimental 2.1. Catalyst preparation and characterization The catalyst was prepared by ion exchange of a commercial zeolite Na-Y (Strem Chemicals Co., Lot.Nr.148960) with an aqueous solution of La(NO3)3 (0.2 mol dm 1, Merck) at 80 8C for 2 h. This ion exchange was repeated once. After filtration, washing with deionized water and drying under ambient conditions, the zeolite was heated in flowing air (50 cm3 min 1) with a rate of 3 8C min 1 up to a final temperature of 400 8C and kept at this temperature for 1 h, before it was allowed to cool down to room temperature. The obtained zeolite was hydrated over a saturated aqueous solution of Ca(NO3)2 until its mass became constant. Two consecutive ion exchange steps with an aqueous solution of NH4NO3 (0.5 mol dm 1, Fluka) at 80 8C for 2 h were applied, followed by another ion exchange with aqueous La(NO3)3 solution as described above. To an aqueous slurry of the obtained zeolite, a solution of Pt(NH3)4Cl2 (Fluka) was added dropwise under vigorous stirring at room temperature. The amount of platinum was 0.4 wt.% (based on the anhydrous zeolite). The elemental composition of the catalyst was determined by ICP-AES (Perkin Elmer Plasma 400) after dissolution in an aqueous solution of HF (10 wt.%). The concentrations of Lewis and Brønsted acid sites were calculated from FT-IR spectroscopy after pyridine adsorption at 200 8C, as described earlier [27], using the extinction coefficients determined by Emeis [28]. Table 1 shows the main physicochemical properties of the catalyst. 2.2. Catalytic experiments and catalyst regeneration The zeolite powder was pressed without a binder and used with a particle size between 0.40 and 0.63 mm. The catalyst was activated in a fixed-bed reactor. To achieve a high dispersion of the noble metal, it was first heated in flowing air (50 cm3 min 1) with a rate of 2 8C min 1 to a final temperature of 300 8C. After 16 h, the gas flow was switched to nitrogen (50 cm3 min 1) for 10 min, then to hydrogen (50 cm3 min 1) for 4 h. After cooling down to Table 1 Physicochemical properties of the 0.4Pt/La-Y zeolite catalyst nSi/nAl 3nLa/nAl mNa/mCatalyst mPt/mCatalyst cBrønsted acid sites (mmol/g) cLewis acid sites (mmol/g) Micropore volume (cm3/g)
2.82 1.05 2.4 10 4.0 10 0.33 0.04 0.263
3 3
R. Klingmann et al. / Applied Catalysis A: General 281 (2005) 215–223 Table 2 Reaction conditions employed during the catalytic alkylation experiments mCatalyst (g) pReaction (bar) TReaction (8C) t = VLiquid phase, Reactor/V˙ Feed (h) WHSV1-Butene (h 1)
1.0 20 75 4 0.5
135 8C, the catalyst was quickly transferred to the alkylation reactor, where it was kept under flowing nitrogen (20 cm3 min 1) at 135 8C for 16 h. The alkylation reactions were carried out in a continuous-flow stirred tank reactor (Microclave, Autoclave Engineers Co., V = 50 cm3) equipped with a fixed annular catalyst basket. After cooling to the reaction temperature of 75 8C, the reactor was first filled with 20 g of liquid isobutane by means of a mass flow controller (Brooks Flomega 5881) under nitrogen pressure (ptotal = 20 bar). Then, the premixed isobutane/1-butene feed (nIsobutane/n1-Butene = 9) was continuously fed to the reactor using the same mass flow controller. Isobutane (99.5 vol.%) and 1-butene (99.5 vol.%) were obtained from Air Liquide GmbH and Linde AG, respectively. The main reaction parameters are summarized in Table 2. The reactor effluent was continuously vaporized by means of a heated needle valve and addition of a nitrogen gas flow (100 cm3 min 1) that was preheated to a temperature of 200 8C. An HP-6890 gas chromatograph equipped with on-line sampling, a flame ionization detector and a capillary column (Supelco Petrocol, l = 100 m, di = 0.25 mm, dfilm = 0.5 mm, stationary phase: polydimethylsiloxane, carrier gas: hydrogen (1 cm3 min 1)) were employed for product analysis. After an alkylation experiment, the reactor was depressurized at reaction temperature. Then it was purged with nitrogen (20 cm3 min 1) for ca. 30 min and allowed to cool down to room temperature. To ensure that the properties of the coked catalyst did not change by exposure to air or moisture, the catalyst was stored under nitrogen before placing it in the regeneration reactor. Regeneration of the catalyst was carried out in a hydrogen flow of 40 cm3 min 1 at a pressure of 15 bar in a high-pressure flow-type apparatus with a fixed-bed reactor as described earlier [29]. Heating rates of 100 8C/h up to the final regeneration temperature (200, 250 or 300 8C) were applied. The final temperature was held for a total regeneration time of 7 h 45 min.
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sum of the weight losses due to the desorption of water and the removal of carbonaceous deposits, was determined by TGA up to 950 8C using a heating rate of 20 8C min 1 (Setaram TG DTA 92). The relative amount of oxygen on the catalyst was obtained by subtracting the relative amount of hydrogen and carbon from the total weight loss. As this amount of oxygen was present on the catalyst in the form of water, the relative amount of hydrogen could be divided into one part bound to oxygen (water) and a second part bound to carbonaceous deposits. Micropore volumes were calculated from adsorption isotherms of nitrogen at 196 8C (Micromeritics ASAP 2010). The samples were pretreated at T = 80 8C and p < 10 8 bar overnight. For the UV–vis measurements, the catalyst pellets were powdered in a mortar and placed in a quartz cell. The spectra were recorded in the reflection mode in the 190–700 nm range with a Perkin Elmer Lambda 16 spectrometer at room temperature. The scanning speed was 480 nm min 1. Before recording the spectra, the catalysts were stored in an argon atmosphere to avoid exposure of the catalyst to air or moisture.
3. Results and discussion 3.1. Deactivation of the catalyst Fig. 1 shows the conversion of n-butene and the composition of the reaction products in dependence of time-on-stream. The double-bond shift of 1-butene into cisand trans-2-butene is not considered in the calculation of the conversion, because this isomerization is fast. Under the conditions employed, complete n-butene conversion occurred for ca. 2 h. The desired isooctanes represent the major product fraction which initially accounts for more than 80 wt.%; their yield decreases monotonously with time-on-stream. The isomer distribution of the C8-fraction is shown in Fig. 2. The main components are 2,3,3-, 2,3,4- and
2.3. Characterization of the coked and regenerated catalyst samples The amount of carbon and hydrogen on the coked catalyst samples was determined using elemental analysis (Elementar Vario EL). The carbon contents are given in wt.% of the fully hydrated sample. The nH/nC ratio was calculated by a method combining thermogravimetric analysis (TGA) and elemental analysis: first, the relative amounts of carbon and hydrogen on the catalyst were determined by elemental analysis. Then the total weight loss of the catalyst, i.e. the
Fig. 1. Conversion and product composition during isobutane/1-butene alkylation on 0.4Pt/La-Y.
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Fig. 2. Composition of the isooctane fraction during isobutane/1-butene alkylation on 0.4Pt/La-Y.
2,2,4-TM-Pn, which initially account for more than 85 wt.% of the C8-fraction. Compared to these three trimethylpentane isomers, the primary product one might expect according to the cationic alkylation mechanism, i.e. 2,2,3-TM-Pn, is formed in minor amounts. The nTM-Pn/nDM-Hx ratio initially amounts to 12.2 and drops slowly as the reaction proceeds. Monobranched octane isomers are formed in negligible amounts only. As soon as the conversion begins to drop, i.e. after ca. 2 h, the selectivity to dimethylhexanes, especially 3,4-DM-Hx, increases. Almost identical compositions of the isooctane fraction during isobutane/n-butene alkylation at ca. 80 8C were found earlier by Weitkamp [9] and Rørvik et al. [30] on Ce-Y zeolite catalysts and by Kirsch et al. on a RE-Y zeolite catalyst [31]. Light ends (C5–C7) are formed with a mass fraction around 10 wt.% (cf. Fig. 1). The yield of heavy ends (C9+) increases in a pronounced manner as the reaction proceeds. No unsaturated products could be detected within the first 3 h on-stream. In Fig. 3a, the carbon content of the catalyst and the micropore volume after termination of the alkylation
Fig. 3. (a) Carbon content and micropore volume of the 0.4Pt/La-Y zeolite catalyst in dependence of time-on-stream. (b) UV–vis spectra of the 0.4Pt/ La-Y zeolite catalyst after different times-on-stream.
experiments at different times-on-stream are shown. Within the period of complete n-butene conversion, i.e. within the first 2 h, the amount of carbon deposited on the catalysts increases only slightly and, in-line with this result, the micropore volume decreases only slowly within this time. After 3 h on-stream, when the period of rapid deactivation begins (cf. Fig. 1), the amount of carbonaceous deposits increases steeply, and a concomitant loss in micropore volume from ca. 0.24 to 0.15 cm3 g 1 is observed. This is at variance to results of Pater et al., who found that the rate of coke formation on the zeolite catalyst during alkylation in a fixed-bed plug-flow reactor is high at the beginning of the reaction and then slows down significantly [19], though their experiments were performed at much lower space velocity and higher paraffin to olefin ratio. A distinctive correlation between the build-up of coke inside the pores of the catalyst and the conversion or selectivity of the alkylation reaction was not found. However, our results are in principal accordance with those of Feller et al. [14]. Comparing the signal intensity of MALDI-TOF spectra of La-X zeolite catalysts, that had been used in isobutane/2-butene alkylation at 75 8C for different times, the latter authors found a steep increase in the signal intensity, which paralleled the amount of carbonaceous deposits on the catalyst, at a timeon-stream where the conversion started to drop. The nH/nC ratio of the deposits on the 0.4Pt/La-Y catalyst was 1.6 after 6 h time-on-stream. This is significantly lower than the value of 2.0 expected for a product formed by butene oligomerization and essentially in-line with earlier studies. Weitkamp and Maixner found an nH/nC ratio of 1.8 by elemental analysis for a La,Na-Y zeolite catalyst that had already entered the oligomerization stage [12], while Feller et al. calculated a formula based on MALDI-TOF spectra of CnH2n 4 for the coke molecules desorbed from a deactivated La-X catalyst with a maximum in the range between C17 and C21, which corresponds to an nH/nC ratio of ca. 1.7 [14]. Unsaturated carbocations formed during the alkylation stage seem to be responsible for the relatively low nH/nC ratios of the deposits. Indeed, such enylic cations could be observed by UV–vis spectroscopy (Fig. 3b). The absorption bands were assigned after Fo¨ rster et al. [32]. The intensity of the band at 305 nm, originating from monoenyl carbocations, increases during the first 3 h on-stream and then remains nearly constant. The intensity of the band at 370 nm representing dienyl carbocations increases only during the first hour on-stream. A small band at 450 nm assigned to trienyl carbocations could only be observed within the early stage of the reaction. The formation of such unsaturated carbocations can be rationalized in terms of consecutive hydrogen transfer reactions [33,34]. Donation of a hydride ion from an olefin to a saturated carbocation results in an unsaturated carbocation and an alkane. To some extent, the unsaturated carbocations desorb as dienes. Further hydride transfer from the dienes to saturated carbocations leads to the formation of dienylic cations. In this way, the deposits formed in the early stage of the alkylation reaction can act as
R. Klingmann et al. / Applied Catalysis A: General 281 (2005) 215–223
efficient hydride donors [35,36], in a similar manner as the acid-soluble oils formed during isobutane/butene alkylation catalyzed by sulfuric acid [37]. Scho¨ llner et al. first postulated the key role of unsaturated carbocations in the deactivation of zeolite catalysts used in the isobutane/butene alkylation [38]. The resonancestabilized unsaturated carbocations are more stable than the tert-butyl cation or other saturated carbocations [39,40]. Desorption of these enylic cations via hydride transfer with isobutane is therefore unlikely. They remain almost irreversibly adsorbed and poison the acid sites. Tert-butyl cations, that stand at the beginning of a new catalytic alkylation cycle, cannot be restored, i.e. the concentration of sites available for the alkylation decreases. It can be seen from Fig. 3b that the concentration of monoenylic carbocations (l = 305 nm) on the catalyst surface increases during the initial 2 or 3 h, i.e. as long as the catalyst maintains a high alkylation activity (cf. Fig. 1). Once the butene conversion starts to drop, only negligible amounts of further enylic carbocations are formed. It has been shown by Pine et al. [41], Jacquinot et al. [42] and de la Puente and Sedran [43] that the rate of hydride transfer on zeolite Y is strongly dependent on the density of acid sites. As the acid site density gradually diminishes due to poisoning by the unsaturated carbocations, the hydride transfer ability of the catalyst decreases. The enylic carbocations are themselves formed by reactions involving hydride transfer, it can hence be argued that these species are formed only as long as the catalyst possesses a sufficiently high hydride transfer activity. Carbocations with two or more double bonds can undergo cyclization and form naphthenic rings with side chains, which can in turn form aromatics in consecutive hydrogen transfer reactions. This could explain the disappearance of the small absorption band at 450 nm at higher times-onstream. It is noteworthy, however, that Paze` et al. [44] found evidence for the cyclization of polyenylic carbocations adsorbed on an acidic zeolite only at temperatures above 300 8C. In our study, no absorption bands of aromatics could be detected within the alkylation stage. After 5–6 h time-onstream, a weak shoulder in the band of the monoenyl carbocations at approximately 260 nm appears, which could originate from aromatic compounds. This was also inferred by several groups who found that the deposits contain bulky organic compounds. Using MALDI-TOF-MS [45] or characterizing the extracted organic phase by GC–MS, UV–vis or 13C NMR spectroscopy after dissolution of the coked catalysts in aqueous HF [18,46], it was concluded that the deposits contain two naphthenic or aromatic rings. In the cationic alkylation mechanism [7], isododecyl cations can be formed by addition of butene to isooctyl cations. These C12-carbocations can undergo type A rearrangements and b-scissions [7], whereby smaller carbocations and olefins are formed. At the low temperatures applied in isobutane/butene alkylation on zeolites, only type A b-scissions can occur, and they lead to olefins with at least
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Scheme 1. Formation of unsaturated carbocations from cracking products.
one tertiary carbon atom. As an example, type A b-scission of the 2,3,4,5,5-pentamethylheptyl-3 cation is depicted in Scheme 1. The fragments formed in the type A b-scission of this cation are 3,4-dimethyl-2-pentene and the 2-methylbutyl-2 cation. These primary moieties can further react in several ways (cf. Scheme 1): (A) alkylation, i.e. the reverse reaction of the preceding cracking, (B) hydride transfer from the olefin to the carbocation resulting in 2-methylbutane and an allylic carbocation and (C) alkylation of a saturated carbocation (R = alkyl) at a neighboring acid site by the olefin or protonation (R = H) of the olefin by an adjacent free Brønsted acid site. As hydride abstraction from tertiary carbon atoms, especially in allylic position, proceeds much easier than from secondary or primary C-atoms of the feed butene molecules [47], it is hence reasonable to assume that the branched olefins formed in cracking reactions are the main precursors for the unsaturated carbocations formed via route (B). The decrease in the reaction rate of the alkylation due to the loss of active sites by poisoning results in a higher olefin concentration inside the catalyst pores. As a consequence, the oligomerization of olefins is more and more favored over the alkylation reaction. As soon as oligomerization becomes the prevailing reaction, high molecular weight hydrocarbons are formed inside the pores of the catalyst. These oligomers are responsible for the steep increase in the coke content of the catalyst and the concomitant decrease of the micropore volume (Fig. 3a). At this time-on-stream olefins appear in the product, i.e. the carbocations formed during alkylation are no longer desorbed via hydride transfer with isobutane, but by back-donation of a proton to the zeolite. 3.2. Regeneration of the catalyst The coked catalysts were regenerated in the fixed-bed reactor in a hydrogen flow at elevated pressure. Three different temperature programs were applied. It is well known from several related processes, such as skeletal isomerization or reforming, that elevated hydrogen pressure
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Fig. 4. Intensity of the FID signal during regeneration of the 0.4Pt/La-Y zeolite catalyst at different temperatures.
is necessary to suppress the formation of coke deposits and catalyst deactivation [48]. Therefore, a hydrogen pressure of 15 bar was chosen for the regeneration. The total flux of organic compounds released from the catalyst during regeneration was detected with a flame ionization detector (FID). In Fig. 4, the FID signal intensity is plotted versus the time of regeneration for a catalyst that had been used for 3 h in the alkylation. Raising the temperature up to 200 8C results in a maximum signal intensity at 200 8C. Thereafter, the intensity drops slowly within the remaining time of regeneration. If a temperature program up to 300 8C is applied, the signal intensity reaches its maximum at 300 8C, whereupon it drops rapidly to zero. No temperatures above 300 8C were applied in the regeneration of the catalyst, since this would have resulted in a diminution of the concentration of Brønsted acid sites [49]. An as high as possible concentration of Brønsted acid sites is known to be favorable for alkylation catalysts [50]. These results are in principal accordance with Querini’s work [51], who also observed a maximum in FID signal intensity at ca. 300 8C in the regeneration of a 0.5 Pt/La,H-Y zeolite catalyst which had been used in the isobutane/nbutene alkylation. Moreover, our results are in-line with studies on hydrocracking of long-chain alkanes on similar zeolite catalysts. For example, it has been reported that, on a Pt/Ca-Y zeolite catalyst, hydrocracking of n-dodecane starts around 250 8C and is complete at 300 8C [26]. Fig. 5a shows the residual carbon contents of the zeolite catalyst after its regeneration at different temperatures. Applying a final regeneration temperature of 300 8C results in a decrease of the carbon content from ca. 3 wt.% before the regeneration to 0.2 wt.% after the regeneration. Lower final temperatures of 200 or 250 8C result in significant higher residual carbon contents. These results support those obtained by observing the total flux of organic compounds released from the catalyst during regeneration (Fig. 4) and show that a regeneration temperature of 300 8C is favorable for removing the coke deposits.
Fig. 5. Carbon content after regeneration of the 0.4Pt/La-Y zeolite catalyst (a) at different temperatures; (b) after different times-on-stream of the alkylation reaction.
Regenerating catalysts after different times-on-stream in the alkylation reaction does not result in markedly different carbon contents after the regeneration at 300 8C (Fig. 5b). This is a remarkable result, because, as shown in Fig. 3a, the amount of carbon on the catalyst does strongly increase with time-on-stream in the alkylation reaction, i.e. before the regeneration. These pronounced differences in the carbon content of the used zeolite catalyst before and after the regeneration must have to do with the composition of the carbonaceous deposits after different times-on-stream: the unsaturated species formed in the early stage of alkylation, as observed by UV–vis spectroscopy, are presumably more difficult to remove by hydrocracking than the long-chain highly branched saturated carbocations formed by olefin oligomerization in the late stage of the reaction. A reason for this could be that the aforementioned unsaturated species can act as precursors for ‘‘hard coke’’ [33]. As these species are mainly formed in the beginning of the alkylation reaction, when the hydride transfer activity is still high, they will be present in similar concentrations on the catalyst after 1, 3 and 5 h of alkylation. The saturated high molecular weight carbocations, which are mainly responsible for the observed increase in carbon content after extended times-onstream in the alkylation, are removed relatively easily by hydrocracking [52]. Fig. 6 shows the distribution of the products obtained during regeneration of the 0.4Pt/La-Y zeolite catalyst, that had been used in the alkylation reaction for 3 h. This distribution changes markedly with increasing temperature. At 200 8C, large amounts of C8 hydrocarbons are released from the catalyst (not shown in Fig. 6). Presumably, these originate from the desorption of physisorbed isooctanes or the displacement of isooctyl cations from the acid sites. Besides isooctanes, cracked products are observed at 200 8. At 300 8C, the products consist predominantly of hydrocracked products, mainly of saturated short-chain molecules, especially butanes. The product distribution very much resembles that obtained by hydrocracking of n-
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Fig. 6. Comparison of the products formed by hydrocracking during regeneration of the 0.4Pt/La-Y zeolite catalyst at 300 8C with the products formed by hydrocracking of n-dodecane on a Pt/Ca-Y zeolite catalyst [26] at 325 8C.
dodecane on a Pt/Ca-Y zeolite catalyst at a similar temperature (Fig. 6) [26]. This can be explained by the fact that after 3 h of alkylation the major part of the coke deposits consists of long-chain saturated oligomers. It appears reasonable to assume that the cracking of these moieties proceeds in a similar manner as that of long-chain alkanes adsorbed from the gas phase [7]. The differences in the product composition may arise from the following fact: During hydrocracking experiments, long-chain hydrocarbons are continuously fed into the reactor. These feed molecules compete with the cracked product molecules for acid sites and thereby suppress the occurrence of secondary cracking reactions [53]. Therefore, it is to be expected that the yield of secondary cracked products in the regeneration surpasses that obtained in the continuous hydrocracking of n-dodecane. In this way, the formation of increased amounts of butanes during the regeneration experiment (Fig. 6), even though it was performed at a slightly lower temperature than the hydrocracking experiments with n-dodecane, may be accounted for. To demonstrate that the hydrogenative regeneration restores the alkylation activity, a catalyst, which had been used for 3 h in the alkylation and subsequently regenerated at 300 8C, was re-used in an alkylation experiment. Both the catalytic activity and the product selectivity (Figs. 7 and 8) were fully restored by the regeneration. Even in a third alkylation reaction following the second regeneration step, the activity and selectivities were essentially equal to those in the first alkylation reaction. The catalyst was characterized after each alkylation and regeneration step using UV– vis spectroscopy (Fig. 9). The intensity of the signals corresponding to monoenylic carbocations decreases slightly with the number of alkylation steps. The reason could be a small decrease of the concentration of acid sites, since these sites are poisoned by coke deposits which cannot be removed during the regeneration. After each regeneration step, UV–vis signals were absent. This shows that polyaromatic species were not formed in detectable
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Fig. 7. Conversion and product composition during isobutane/1-butene alkylation on the 0.4Pt/La-Y zeolite catalyst within three alkylation/regeneration cycles (at the times indicated by the vertical dotted lines, the catalyst was regenerated at 300 8C).
Fig. 8. Composition of the isooctane fraction during isobutane/1-butene alkylation on 0.4Pt/La-X within three alkylation/regeneration cycles (at the times indicated by the vertical dotted lines, the catalyst was regenerated at 300 8C).
Fig. 9. UV–vis spectra of the 0.4Pt/La-Y zeolite catalyst after alkylation and after regeneration within three alkylation/regeneration cycles.
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amounts. However, it cannot be excluded that a slow buildup of such species would have resulted in a gradual deterioration of the alkylation catalyst, if a larger number of alkylation/regeneration cycles had been carried out.
4. Conclusions The formation of enylic cationic species seems to be a decisive step in the deactivation of zeolitic catalysts during isobutane/butene alkylation. These unsaturated carbocations are presumably formed via hydride transfer from olefins originating from cracking of C12+ or higher carbocations. The enylic carbocations remain adsorbed almost irreversibly and poison the acid sites of the catalyst. Thereby, the density of acid sites available for catalyzing the desired alkylation reaction decreases steadily. The resulting higher olefin concentration leads to a rapid deactivation of the catalyst via oligomerization and subsequent blocking or filling of the zeolite pores. Our regeneration experiments have demonstrated that the catalyst activity can be completely restored by applying a temperature program up to 300 8C at a hydrogen pressure of 15 bar without any loss in selectivity to the desired trimethylpentanes. After three cycles of alkylation/regeneration, no absorption bands of polyaromatic compounds were observed in the UV–vis spectrum. Thus, UV–vis spectroscopy can be used as a powerful tool for characterizing the efficiency of the regeneration of zeolite catalysts coked in the alkylation of isobutane with butenes. Glossary DM-Hx: dimethylhexane FID: flame ionization detector FT-IR: Fourier transform infrared ICP-AES: atomic emission spectroscopy by inductively coupled plasma MALDI–TOF: matrix assisted laser desorption ionization–time of flight MS: mass spectroscopy TM-Pn: trimethylpentane PFR: plug flow reactor
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