Ind. Eng. Chem. Res. 1997, 36, 3027-3031
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Role of Lewis Acidity in the Deactivation of USY Zeolites during 2-Methylpentane Cracking Scott M. Babitz, Mark A. Kuehne, and Harold H. Kung* Ipatieff Laboratory and Department of Chemical Engineering, Northwestern University, Evanston, Illinois 60208-3120
Jeffrey T. Miller Amoco Oil Company, P.O. Box 3011, Mail Station H-9, Naperville, Illinois 60566-7011
The role of strong Lewis acid sites in the deactivation of H-USY zeolite during 2-methylpentane cracking was investigated. NH3 adsorption microcalorimetry and FTIR of adsorbed NH3 were used to characterize the acid sites of an H-USY zeolite and another USY sample in which the strong Lewis acid sites were poisoned with ammonia. It was found that poisoning of the Lewis acid sites did not affect the rate of deactivation, the cracking activity, and the distribution of the cracked products. Thus, strong Lewis acid sites are not important in the cracking reaction. Introduction Fluid catalytic cracking is a key step in the production of gasoline. It is believed to be catalyzed by Brønsted acid sites present in the zeolite and involves carbenium ion like intermediates. Industrially, this reaction is catalyzed by the hydrogen form of ultrastable Y (H-USY) zeolites that are formed by steaming Y zeolites. It is well documented that steaming results in large increases in the catalytic activity (Lunsford, 1991) and changes in the zeolite properties (Choi-Feng et al., 1993; Beyerlein et al., 1991; Ge´lin and Courie`res, 1991; Fejes et al., 1988). In particular, dealumination of the zeolite framework occurs, with the concurrent generation of extraframework Al species (AlEF), mesopores, and fractures in the zeolite crystallites. The forms of AlEF and their role in the cracking reaction are not well understood. It is established that at least some AlEF form strong Lewis acid sites that adsorb basic molecules, such as NH3 and pyridine, with high heats (Aroux and Ben Taarit, 1987; Shi et al., 1988; Aroux et al., 1994; Chen et al., 1992). It has also been proposed that the Lewis acid sites could withdraw electron density from a neighboring framework lattice oxygen of protonic sites, thereby enhancing the strength of these Brønsted acid sites (Lunsford, 1991; Mirodatos and Barthomeuf, 1981). The formation of these enhanced Brønsted sites has been used to explain the increase in the cracking activities (Sohn et al., 1986; Wang et al., 1991; Lunsford, 1991). Alternatively, it has also been proposed that the strong Lewis acid sites of AlEF promote hydride abstraction and help initiate the cracking reaction (Abbot, 1989). There are conflicting data regarding this latter proposal; extraction of AlEF from H-USY lowered the gas oil cracking activity (Corma et al., 1991), but increased the 2-methylpentane cracking activity (Bamwenda et al., 1995). The H-USY zeolite is rapidly deactivated during hydrocarbon cracking by the formation of coke. In view of the potentially important effects of AlEF on cracking activity, it has been of interest to elucidate the role of AlEF in the deactivation process. By comparing the gaseous products of the cracking reaction on zeolites containing different ratios of Brønsted/Lewis acid sites, it was suggested that Lewis acid sites accelerate the * Author to whom correspondence should be addressed. S0888-5885(96)00620-3 CCC: $14.00
chain process of cracked products on the catalyst that leads to coke formation (Abbot, 1990; Abbot and Guerzoni, 1992). The presence of Lewis acid sites has also been shown to increase the coke content of a deactivated zeolite (Moljord et al., 1995), enhance the rate of deactivation (Wang et al., 1991), or produce coke of a higher degree of unsaturation (Flego et al., 1995). Recently, we compared the cracking activities and the acidic properties of an H-USY and an (H,NH4)-USY sample (Kuehne et al., 1997). These two samples differed in that the strong Lewis acid sites of (H,NH4)USY were poisoned by ammonia. The results showed that these strong Lewis acid sites had very little effect on the cracking activity of the H-USY zeolitesthe activities of the two samples were very similar. In view of this finding, it became interesting to examine the role of Lewis acid sites in the deactivation process using these samples, that is, by comparison of the deactivation characteristics of the H-USY and the (H,NH4)-USY. This paper reports the results of this investigation. Experimental Section H-USY zeolite was prepared by the calcination of commercial NH4-USY (UOP, LZ-Y84) in air at 723 K for 16 h. It had a unit cell size of 24.468 Å, which corresponded to 26 framework Al per unit cell by using the correlation: AlF/unit cell ) 112.4 (ao - 24.233) (Fichtner-Schmittler et al., 1984). It had a surface area of 726 m2/g and a micropore volume of 0.282 cm3/g and contained 0.12 wt % (0.05 mmol/g) Na determined by ICP AA spectroscopy. The (H,NH4)-USY sample was prepared from H-USY as follows. The H-USY sample was ammonium ion exchanged three times in NH4NO3, washed two times with deionized water, and then washed a final time with a solution of NH4OH (pH ) 9.5). The sample contained 0.014 wt % (0.006 mmol/g) Na. Then the sample was calcined in situ for 16 h at 573 K in N2 flow prior to 2-methylpentane cracking or in vacuum before Fourier transform infrared spectroscopy (FTIR) and microcalorimetry analysis. After this calcination step, temperature-programmed desorption (TPD) analysis indicated that 0.323 mmol/g of NH3 remained on the sample. In addition, 2-methylpentane cracking data indicated that the calcination method (N2 flow or vacuum) had no effect on the performance or deactivation characteristics of the catalyst. © 1997 American Chemical Society
3028 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997
The acid sites of these samples were characterized by microcalorimetry and FTIR of NH3 adsorption as previously described (Kuehne et al., 1997). Briefly, for NH3 adsorption microcalorimetry, the zeolite powder was pressed at 39 MPa into a wafer and then broken into 20/40 mesh pieces. The sample was heated under vacuum at 573 K for 16 h and then cooled to 473 K for measurement. For FTIR measurements, the zeolite powder was pressed at 190 MPa into a wafer. The wafer was then heated under vacuum at 573 K for 16 h. Afterward, the sample was cooled to 473 K for NH3 adsorption and spectrum collection. FTIR data were collected at several surface coverages of ammonia. A measured amount of ammonia gas was dosed onto the sample before each scan. The spectra presented here are the ratio of a zeolite wafer with a specific surface coverage of ammonia (sample scan) against a fresh zeolite wafer (a background scan, before dosing on ammonia). It should be emphasized that the only IR adsorption bands of ammonia measured were those from ammonia that were dosed onto the zeolite during the IR experiment. With the (H,NH4)-USY sample, ammonia derived from the ammonium ions from ion exchange was present in both the sample and background scans. When a ratio of these two scans was made, the difference in the ammonia content, solely attributed to the ammonia dosed on to the sample during the experiment, was measured. The cracking activity of 2-methylpentane was measured in a flow microreactor at 573 K. The zeolite powder was pressed and then made into 50/80 mesh pieces. About 0.02-1.5 g of zeolite was mixed with an amount of 30/50 mesh R-Al2O3 sufficient to make the total bed volume 1.4 cm3 in the flow reactor. An additional 0.3 cm3 of R-Al2O3 was added to the top of the catalyst bed to preheat the feed gases. The H-USY sample was pretreated in flowing nitrogen in the reactor at 573 K for 0.5 h prior to use. The final preparation stage of the (H,NH4)-USY sample (heating in flowing nitrogen at 573 K for 16 h) was conducted in the reactor immediately prior to use. After the pretreatment, the gas stream was switched to a stream of N2 saturated with 2-methylpentane at 273 K to begin the reaction run. The reaction products were measured at several times on stream over a 1.5-2 h period using an on-line gas chromatograph (GC) with a flame ionization detector (FID). An FID response factor of 1.0 was used to calculate percent weight selectivities for all hydrocarbon species (Dietz, 1967). In earlier experiments products were separated in the GC by a 10 ft 1/8 in. stainless steel column packed with 80/100 mesh n-octane/Porasil C (Alltech) and a temperature program: 5 min at 80 °C, and then at 20 °C/min from 80 °C to 145 °C. In later experiments, a cross-linked, 5% PHME siloxane capillary column was used (Hewlett-Packard, 30 m × 0.32 mm, with 0.25 µm film thickness, split ratio ) 50:1, at 303 K, column flow rate ) 2.6 mL/min). The cracking conversion was defined as conversion to all hydrocarbon products except 2-methylpentane isomers. C6 alkanes, which were isomers of 2-methylpentane, were excluded in the calculation because they were formed primarily by isomerization. It has been shown that isomerization is a process of lower activation energy (Zhao et al., 1993; Daage and Fajula, 1983). Thus, under our reaction conditions, the rate of isomerization was much greater than the rate of cracking, and at short contact times (W/F) the products from isomerization reactions would dominate. This is illustrated
Figure 1. Initial product selectivities over H-USY, for conversion to all reaction products (cracking and isomerization) as a function of W/F. (C6s excludes 2-methylpentane).
Figure 2. ∆H of NH3 adsorption at 473 K of (a) H-USY, (b) (H,NH4)-USY, and (c) (H,NH4)-USY heated to 723 K for 10 h.
in Figure 1, which shows that the initial hydrocarbon products were predominantly 2-methylpentane isomers (C6s) at short contact times. Results Characterization of Acid Sites. The differential heat of NH3 adsorption on H-USY at 473 K is shown in Figure 2. In agreement with literature results, the heat decreased with increasing NH3 coverage, from an initial heat of >140 kJ/mol to about 90 kJ/mol at a coverage of 1.4 mmol/g. This latter coverage compared well with 1.6 mmol/g of acid sites determined by NH3 TPD and was lower than 2.3 mmol/g of framework Al estimated from the unit cell size of the zeolite (XRD). The difference between the number of acid sites found from XRD and the calorimetry measurements, or XRD and TPD, has been observed previously in H-USY and steamed zeolites (Parrillo and Gorte, 1993; Aroux et al., 1994). This difference could be attributed to the neutralization of some of the Brønsted acid sites by extraframework Al species. Adsorption of 0.8 mmol/g of NH3 at 473 K was on both Brønsted and Lewis acid sites, as illustrated by the FTIR spectrum of the adsorbed species shown in Figure 3 (spectrum a). A peak at 1440 cm-1 due to adsorption on Brønsted acid sites and peaks at 1625 and 1316 cm-1 due to the Lewis acid sites were observed (Kosslick et al., 1994). Since desorption of NH3 from the Lewis acid sites required
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Figure 4. 2-Methylpentane cracking conversion as a function of time on stream. The W/F for H-USY was 29 g‚s/mmol, and for (H,NH4)-USY it was 25 g‚s/mmol. Figure 3. FTIR spectrum of adsorbed NH3 at 473 K and a coverage of about 0.8 mmol/g: (a) H-USY, (b) (H,NH4)-USY, and (c) (H,NH4)-USY heated to 723 K for 10 h.
the highest temperature, it has been suggested that the initial high heats were due to adsorption on Lewis acid sites (Aroux and Ben Taarit, 1987; Chen et al., 1992). The initial heat of adsorption of NH3 on (H,NH4)-USY was only about 128 kJ/mol, a value substantially lower than that on H-USY, as shown in Figure 2. The heat decreased slowly to about 121 kJ/mol at a coverage of 0.8 mmol/g. Throughout this coverage range, the FTIR spectrum of the adsorbed NH3 showed that adsorption occurred only on the Brønsted acid sites (1440 cm-1 peak). There was no evidence of adsorption on Lewis acid sites. At higher coverages, the heat decreased more rapidly in a manner very similar to that for H-USY to 90 kJ/mol at 1.4 mmol/g. Thus, all the strong Lewis acid sites on the (H,NH4)-USY sample were covered by the 0.323 mmol/g of NH3 in the sample as synthesized. This left only Brønsted acid sites for NH3 adsorption during the FTIR experiment. In order to establish that the procedure to prepare (H,NH4)-USY did not cause permanent changes in the zeolite, such as dissolution of extraframework Al species, the final step of the sample preparation, the sample calcination, was also performed at 723 K. TPD analysis showed that calcination at this higher temperature removed the 0.323 mmol/g of NH3 that was left on the sample after calcination at the lower temperature (573 K). The acid strength distribution of the resulting sample was determined by microcalorimetry, and the result is shown in Figure 2. Within experimental error, the differential heat curve was identical to that of H-USY. Similarly, the FTIR spectrum of the adsorbed NH3 on this sample showed adsorption on both Lewis and Brønsted acid sites (Figure 3, spectrum c). These results show that the sample (H,NH4)-USY was the same as sample H-USY except that, in the former, the strong Lewis acid sites were covered with adsorbed NH3. 2-Methylpentane Cracking. The activity of both H-USY and (H,NH4)-USY declined with time on stream. As shown by the data in Figure 4, the decrease in conversion followed the empirical equation:
X ) X0 exp(-kdt0.4)
(1)
where X is the fractional conversion at time on stream
Figure 5. Initial cracking conversions of 2-methylpentane over (a) H-USY and (b) (H,NH4)-USY as a function of W/F.
(t), and X0 is the conversion at time t ) 0. This correlation is purely empirical and was chosen as it fit the data well. However, kd is a measure of the deactivation rate, and X0 is a measure of the catalytic activity of a fresh sample before deactivation. These are meaningful parameters for comparison among samples. Table 1 shows the values of kd for the two samples. From these values a linear plot (Figure 5) of -ln(1 X0) against (W/F) could be constructed, the slope of which was the rate constant per unit weight of catalyst, k. These rate constants are also listed in Table 1. As explained previously, C6 isomers were excluded from the calculation of fractional conversion. These products are primarily from isomerization and not cracking reactions. The data in Figures 5 and 6 and Table 1 clearly show that the presence of adsorbed NH3 on H-USY, strongly held at the Lewis acid sites, has very little effect on the catalytic cracking properties of the USY zeolite. The
3030 Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 Table 1. Values for kd and k for 2-Methylpentane Cracking over H-USY and (H,NH4)-USYa catalyst
kd (s-0.4)b
k (µmol/g‚s)
H-USY (H,NH4)-USY
0.33 ( 0.03 0.32 ( 0.03
10.1 ( 2.2 10.1 ( 2.2
a Approximate flow rates: 2-methylpentane flow rate, 3 sccm; total flow rate (2-MP in N2), 40 sccm (sccm ) cm3/min at 101 kPa and 273 K). Reaction temperature: 573 K. b Average value over experiments with W/F from 13 to 40 g‚s/mmol; there was no apparent correlation between kd and W/F.
Figure 6. 2-Methylpentane cracking product selectivities as a function of time on stream: (a) H-USY and (b) (H,NH4)-USY. (Each plot combines the results for three runs performed with that catalyst in the range W/F ) 13-40.)
cracking rate constant of the (H,NH4)-USY sample is identical to that of the H-USY sample, and the deactivation rate constants and the product distributions (Figure 6) are very close for the two samples. These results show that strong Lewis acid sites, which are developed by the steaming of HY to form H-USY and adsorb NH3 with heats higher than about 130 kJ/mol, are not important in either the rate of cracking or the deactivation kinetics during cracking. In fact, the data show that strong acid sites (either Brønsted or Lewis acid sites) are not important in the cracking reaction. The sodium content of the (H,NH4)-USY (0.006 mmol/ g) was substantially lower than that of H-USY (0.05 mmol/g). However, this difference had a negligible effect on the cracking activity, as shown by a comparison of the activities of H-USY and the (H,NH4)-USY calcined at 450 °C. Calcining at this temperature removes all the ammonia, and the activity of the latter sample was the same as that of H-USY (Kuehne et al., 1997).
Discussion There are literature results that suggest an active role of Lewis acid sites of extraframework Al species in the deactivation process. In most of these studies, the conclusions are based on differences in the coke material or the cracked gas phase products between zeolite samples containing different amounts of Lewis acid sites. Abbot and Guerzoni (1992) compared H-mordenite samples that contained different ratios of Brønsted to Lewis acid sites prepared by pretreating the zeolite to different temperatures. They found that the alkane/ alkene ratio in the products of n-octane cracking was above unity and higher for the sample with a higher fraction of Lewis acid sites. Since a larger alkane/ alkene ratio in the product stream implies the formation of more unsaturated adsorbed species, this suggests that Lewis acid sites promote coke formation. Similarly, the correlation of Lewis acidity with decline in H2 production was interpreted as evidence that Lewis acid sites accelerate the chain process of cracked products on the surface that leads to coke formation (Abbot, 1990). The presence of Lewis acid sites might also affect the coke content in a deactivated zeolite. It has been reported that a Y zeolite with a large amount of AlEF contained more coke after deactivation than one with much less AlEF (Moljord et al., 1995) or deactivated faster (Wang et al., 1991). Spectroscopic investigation of the coke formed on LaHY-FAU showed that on a sample that had been activated to a higher temperature, and thus contained more Lewis acid sites, the adsorbed hydrocarbon species contained a higher degree of unsaturation (Flego et al., 1995). Finally, Lewis acidity has been suggested to contribute to the difference in the deactivation characteristics of H-USY, H-ZSM-5, and H-mordenite (Niu and Hofmann, 1995). On the other hand, it has been reported that a substantial amount of Lewis acid sites could still be detected on a deactivated HY sample, even though the concentration of Brønsted acid sites was substantially decreased (Jolly et al., 1994). Unfortunately, in that study, it was not established whether the detected Lewis acid sites were strong or weak. It is possible that the different conclusions from these investigations are due to the fact that the measurements did not make direct comparison among zeolite samples that were otherwise identical but differed only in the presence of Lewis acid sites. In some cases, different types of zeolites were used (Niu and Hofmann, 1995); whereas in others, the zeolites differed in the extent of leaching (Moljord et al., 1995; Wang et al., 1991; Bamwenda et al., 1995) or concentration of Brønsted acid sites (Abbott and Guerzoni, 1992; Flego et al., 1995). In these investigations, other factors, such as pore size or pore blockage by AlEF species, may contribute to the differences in deactivation among samples, in addition to the presence of Lewis acidity. In this study, a direct comparison is made using the same H-USY sample by poisoning the strong Lewis acid sites. The results provide direct evidence that these strong Lewis acid sites did not affect the deactivation characteristics or the cracking activity of the catalyst. Acknowledgment Financial support of this work by the National Science Foundation and in kind support by Amoco Corporation are gratefully acknowledged. We also acknowledge the
Ind. Eng. Chem. Res., Vol. 36, No. 8, 1997 3031
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Received for review October 4, 1996 Revised manuscript received February 4, 1997 Accepted February 4, 1997X IE9606202
X Abstract published in Advance ACS Abstracts, June 15, 1997.
