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Hydroisomerization of n-Heptane over Pt-Loaded USY Zeolites. Effect of Steaming, Dealumination, and the Resulting Structure on Catalytic Properties Raed H. Abudawood, Faisal M. Alotaibi, and Arthur A. Garforth* School of Chemical Engineering and Analytical Science, The University of Manchester, Manchester, M13 9PL, U.K. ABSTRACT: The hydroconversion of n-heptane was studied on five USY zeolite samples loaded with 1 wt % Pt. Experiments were performed on a continuous fixed-bed stainless steel reactor at 210310 °C and pressures up to 15 bar. Three in-house samples were subjected to steaming treatment, and the remaining were acid-leached from a commercial source. The study aimed to evaluate the effect of acid leaching, and its subsequent extraframework alumina (EFAL) species removal, on the performance of bifunctional USY zeolite catalysts during n-heptane transformation. Results have shown that steamed samples generate more cracked products at higher conversions when compared to acid-leached ones, more than likely due to a high presence of EFAL species. The degree of steaming played a role in decreasing cracking tendency at higher pressures, which is attributed to pore structure change and decreasing acidity. In addition, steaming resulted in a catalyst sample capable of generating isomers with blended research octane numbers close to those achieved with a robust commercial catalyst. However, poisoning experiments have shown that these two catalysts are highly sensitive to sulfur and require sulfur-free feeds in order to demonstrate their full capacities.
’ INTRODUCTION Stricter environmental regulations imposed worldwide on gasoline automotive fuel quality have left refiners facing a challenge: the content of environmentally unfriendly aromatics in gasoline fuel must be reduced13 with the fuel still meeting the minimum combustion efficiency requirement, expressed by its research octane number (RON).46 Finding a high-octane substituent for aromatics is not a straightforward task. For instance, main octane enhancers blended in gasoline have either been completely phased out due to their toxicity, such as in the case of lead-containing additives, or seen their use decline due to environmental concerns, like oxygenates such as MTBE.79 Moreover, adding high-octane reformates to gasoline fails to do the trick, since aromatics are a major constituent of reformates.1012 A potential solution to this problem has been proposed in the hydroisomerization of lower-value refinery heavy naphtha streams to improve their RON. This will enable the blending of higher amounts of heavy naphtha into gasoline, reducing the need for additives in formulation. The application of specific bifunctional zeolite catalysts in upgrading the RON of refinery light naphtha streams through hydroisomerization has already been commercialized.1315 This application, however, has not yet been expanded commercially to include the processing of heavy naphtha, which typically contains normal alkanes from the range of heptane to decane.1618 In the present work, n-heptane (RON = 0) was chosen as a model compound to represent heavy naphtha in studying the hydroisomerization reaction over bifunctional USY zeolite catalysts. Zeolites are crystalline aluminosilicates19,20 that have unique pore structures and catalytic properties, promoting their use in many industries, most importantly in refining.21,22 Zeolite Y can be hydrothermally treated with steam to form its ultrastable counterpart USY.23 Steaming treatments introduce extraframework alumina (EFAL) species, and these species are responsible for the presence of Lewis (L) acid sites.24,25 r 2011 American Chemical Society
Hydroisomerization of normal alkanes proceeds ideally by the bifunctional mechanism in which a noble metal, usually platinum, provides the hydrogenation/dehydrogenation functions of the catalyst, and a zeolitic support provides the acidic function responsible for carbenium ion rearrangements.26 As stated, the presence of EFAL species has an impact on their acidic properties, leading to a modified selectivity. Wang, Giannetto, and Guisnet27 reported that a higher presence of EFAL species contributed to a higher cracking activity and coking rate. They noticed also that EFAL species positively affect the isomerization activity of catalysts, possibly due to the inductive effect that the Lewis acid sites potentially have on existing Brønsted (B) sites. This inductive effect has been highlighted by Remy et al.,28 who found that deep dealumination of USY samples by acid leaching and the removal of most of the EFAL species from catalyst results in smaller turnover frequencies (TOF) over framework aluminum atoms (Al(IV)F), suggesting the inductive effect. In this work, we examined the effect of different dealumination treatments on Pt-loaded USY catalyst properties, namely, activity, selectivity, and stability. In addition, the effect of reaction conditions on the yield of isomers, RON, and product distribution was studied. Catalyst sensitivity to feed sulfur was also tested.
’ EXPERIMENTAL SECTION The three in-house USY samples were generated as follows: a parent Y zeolite material with a sodium content of 1.05 wt % was steamed at 425 °C and then underwent a mild ion-exchange treatment with ammonium sulfate (1 M) at 70 °C for 30 min, resulting in a drop in sodium content to 0.24 wt %. The resulting sample was used to generate the three USY samples: first, by Received: March 7, 2011 Accepted: June 20, 2011 Revised: May 27, 2011 Published: June 20, 2011 9918
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Table 1. Properties of Pt/USY Samples Na contentb
Al per
BET surface
pore
average pore
Brønsted/
metal
crystallinitya (%)
(wt %)
bulk Si/Alc
unit celld
areae (m2/g)
volumee (cm3/g)
diametere (Å)
Lewis acid sitesc
dispersionf (%)
USY-A
80.6
99% purity (Sigma-Aldrich) was fed to the catalyst bed using an HPLC pump, and hydrogen of high purity was introduced to give an H2/HC molar ratio of 9. Catalytic runs were carried out at temperatures from 210 to 310 °C to achieve comparable conversions for all catalysts. Overall pressure ranged from 1 to 15 bar, and space time ranged from 35.14 to 140.6 kg 3 s/mol. Products were analyzed using a Varian 3400 GC equipped with a 50 m 0.32 mm i.d. PLOT Al2O3/KCl capillary column fitted to an FID detector for gas products and a CP-Sil PONA CB optimized gasoline column fitted to an FID detector for liquid products. For the poisoning experiments, 1 wt % and 100 ppm solutions of dimethyl disulfide (DMDS) (equivalent to 0.64 wt % and 64 ppm sulfur) were prepared in n-heptane for
Figure 1. Overall conversion of n-C7 (T = 230 °C, P = 1 bar, W/Fo = 70.6 kg 3 s/mol except CBV-760 where the results shown are at 290 °C with a space time of 140.6 kg 3 s/mol).
tests on USY-C and CBV-712, respectively at 210 °C, atmospheric pressure, and a space time of 140.6 kg 3 s/mol.
’ RESULTS I. Deactivation Behavior. The deactivation behavior of all samples was studied at 230 °C, atmospheric pressure, and a space time of 70.6 kg 3 s/mol. Overall conversion of n-heptane was calculated either in moles or in weight percent as
X ¼
ðnC7 Þ0 ðnC7 Þf 100 ðnC7 Þ0
ð1Þ
where (nC7)0 and (nC7)f are the initial and final concentrations of n-heptane, respectively. Figure 1 shows the overall conversion of n-heptane as a function of time on stream. CBV-760 was not active at this temperature, and the results are at 290 °C and a space time of 140.6 kg 3 s/mol. Sample USY-A showed a very rapid deactivation, possibly due to its high acidity and low average pore diameter, which might have caused excessive cracking and potentially diffusion limitation inside its pores. Further testing of USY-A was halted. Despite the initial sharp drop in activity for USY-B from around 70 to 55 wt % conversion, its activity stabilized quicker than the remaining catalysts, whose activities continued to decline throughout the course of experiments. It is worth mentioning that USY-B has the largest pore volume and might suggest a lessening in diffusion limitations. In order to test for repeatability, the overall conversion for sample USY-B was measured both before and after air regeneration to burn any deposited coke. The aim was to evaluate whether 9919
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Figure 2. Overall conversion of n-C7 before and after air regeneration for USY-B (T = 210 °C, P = 1 bar, W/Fo = 70.6 kg 3 s/mol).
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regeneration restored the catalyst’s initial activity. Figure 2 shows the overall conversion of n-heptane over sample USY-B, both before and after coke removal, at 210 °C, atmospheric pressure, and a space time of 70.6 kg 3 s/mol. It is noted from Figure 2 that the overall conversion in the repeat experiment was slightly less than that in the initial one. It could be argued that the observed small differences in conversion values are within experimental error ((5%) and that the activity of the catalyst was restored to a great extent. II. Product Yield and RON. All possible isomers produced during catalytic testing were identified using GC and GCMS techniques. After steady conversion levels were reached, product selectivities for monobranched isomers (MB), multibranched isomers (MuB), and cracking products (CR) were calculated and plotted against n-heptane conversion, as shown in Figure 3. Among the four catalysts, USY-B generated the most cracking
Figure 3. Product selectivities to monobranched isomers (MB), multibranched isomers (MuB), and cracking products (CR) at steady conversion. 9920
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products, with the yield of cracking products exceeding that of multibranched isomers at most conversion levels. This is indicative of a poor acidmetal balance, also suggested by the low metal dispersion. A proper balance would be expected to generate products in a successive manner as shown in eq 2: n-C7 T MB T MuB T CR
ð2Þ
On the other hand, an improper balance would lead to simultaneous production of MB, MuB, and CR, as can be seen in eq 3:
The successive generation of product categories shown in eq 2 is characteristic of ideal bifunctional catalysts, an observation made by Guisnet et al.30 They found that, for a proper balance between acidic and metal functions to exist, the ratio of accessible platinum sites to acidic sites, expressed by nPt/nA, for the hydroisomerization of n-heptane over platinum-loaded Y zeolite must exceed 0.17, a value also confirmed later for n-decane hydroisomerization by Alvarez et al.31 Guisnet et al. reported that, at lower values than 0.17, catalysts generated monobranched, multibranched, and cracking products simultaneously from n-heptane. Despite USY-C showing nonideal behavior at 1 bar, its performance seemed to approach ideal bifunctional behavior as pressure increased, almost matching that of CBV-712. Cracking on USY-C seemed to be suppressed at higher pressures. In contrast, CBV-760 appeared to generate more cracking products as pressure increased, most probably due its considerably lower average pore diameter, which could have promoted cracking within its pores as its structure became increasingly more restricted to the passage of bulky molecules at elevated pressures. In order to have a more realistic view of the selectivities of these USY samples, the RON of blended product isomers has been estimated by a method developed by Nikolaou et al.,32 where the influence of individual hydrocarbon products is taken into account when calculating the total blended RON for the product stream. Estimated blended RON values were plotted against n-heptane conversion and are shown in Figure 4. It appeared that USY-B failed to show improvement in product RON upon rising pressure, with the value reaching a plateau around 17 at 15 bar, whereas the maximum RON for USY-C increased from around 23 to above 30 as the pressure increased from 1 to 15 bar. USY-C seemed to behave differently from other catalysts, since they generated lower RON values with increasing pressures. This decrease is the expected outcome since the selectivity to multibranched isomers, which have high RON contributions, is negatively impacted the higher the pressure.33 Nevertheless, sample CBV-712 superbly maintained RON values close to or above 30 at all tested pressures. III. Effect of Sulfur. The results of the two poisoning experiments are shown in Figure 5. It seems that both tested samples are very sensitive to sulfur in the feed, since they undergo sharp drops in conversion once the feed is switched from pure n-C7 to nC7 plus DMDS. It is worth noting that pure n-C7 has a total sulfur content of 4.6 ppm. The drop in activity of USY-C is greater due to the much higher level of poisoning. However, activity partially recovered upon resumption of n-C7 feed in both cases, though to a lesser degree for USY-C. This behavior is a similar that reported by Romero et al.34 for the hydroisomerization of n-decane over
Figure 4. Blended RON values at steady-state conversion.
Pt-loaded mordenite. However, mordenite appeared more resistant as USY-C lost almost 80% of isomerization activity (I) versus that of cracking (C), as shown by the I/C ratio (Romero et al. quoted a 67% reduction in mordenite). The drop in the I/C ratio was less for CBV-712 as it was only treated with 100 ppm DMDS. The monobranched/multibranched ratio was less affected, indicating that both categories of isomers have a larger dependence on metal sites for their formation. IV. Analysis of Spent Samples. Analysis of spent catalysts was performed to determine their crystallinity and coke content. Crystallinities were determined by XRD and were calculated as a percentage of the crystallinities determined for the fresh NH4-form catalysts. Table 2 lists the crystallinities of all samples tested. Although only tested for a short time (150 min), the crystallinity of USY-A reduced by about 20%. The rest of the catalysts were tested for almost equal duration (900 min). Both in-house catalysts lost more of their original crystallinities (averaging around 50%) when compared to the acid-leached catalysts (averaging around 70%). It should be noted that the XRD analysis was performed on samples without regeneration and thus coke might contribute to the observed loss of crystallinity. Coke content was measured by thermogravimetric analysis (TGA) for the top, middle, and bottom portions of the catalytic bed (typically 5 cm). The TGA analysis for all catalysts is shown in Table 3. The deactivation of USY-A can be accounted for by the rapid buildup of coke in all zones of the catalyst bed due to the high 9921
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Figure 5. Effect of sulfur addition on conversion, monobranched/multibranched C7, and I/C ratio.
Table 2. Crystallinity of Spent Samples sample
framework Si/Al
Table 3. Coke Content of Spent Samples crystallinity (%)
USY-A
5
80
USY-B
79
52
coke content (wt %) sample
top
middle
bottom
USY-C
910
50
USY-A
1.90
1.20
0.80
CBV-712
12
67
USY-B
2.08
0.72
0.22
CBV-760
60
73
USY-C
0.99
1.09
1.10
CBV-712
0.08
0.02
0.06
CBV-760
0.25
0.14
0.14
density of acid sites. Although a similar trend was observed for USY-B, its deactivation was much less, suggesting that more excessive steaming had stabilized the catalyst to some degree by reducing the number and type of active sites. For the most severely steamed in-house catalyst USY-C, very uniform coke distribution (1 wt %) was observed throughout the bed. The very low coke contents observed in CBV-712 and, to a lesser degree, CBV-760 suggests that the EFAL species have promoted cracking and hence the increased production of coke precursors.
’ DISCUSSION Selectivities were carefully chosen so that they reflect a catalyst’s performance at an overall conversion as close as possible to that achieved by other catalysts at the same pressure. Table 4 lists product distributions, RON, and selectivities highlighting key product ratios for each catalyst at 1, 8, and 15 bar.
At atmospheric pressure, the acid-leached samples are more selective than the steamed-only ones, with CBV-712 being the most selective to isomers. USY-B was less selective than others due to its high cracking yield. USY-C was less selective because it generated other products, which were mainly cyclic compounds resulting from heptane dehydrocyclization reactions. These dehydrocyclization products were found in USY-B and USY-C, suggesting that the high presence of EFAL species (higher Lewis acidity) in these two samples might be the cause of their higher dehydrogenation activities. At 8 and 15 bar, USY-C was the most selective catalyst, with both cracking and dehydrocyclization activities greatly suppressed. Cracking selectivity generally decreased for all catalysts, except for CBV-760, whose cracking almost doubled. This might be explained by increased reactant 9922
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Table 4. Selectivity, Product Distribution, RON, and Key Ratios at Comparable Conversions catalyst
USY-B
USY-C
CBV-712
CBV-760
USY-B
USY-C
CBV-712
CBV-760
USY-B
USY-C
CBV-712
CBV-760
P (bar)
1
1
1
1
8
8
8
8
15
15
15
15
conversion (mol %)
30.5
28.4
31.2
31.0
16.5
12.2
14.6
12.7
15.9
14.0
18.0
15.6
T (°C)
252.0
214.0
252.0
311.8
213.0
233.8
211.4
292.2
215.0
233.0
231.0
296.0
hydroisomerization
84.5
85.9
93.5
89.4
87.2
95.0
93.6
79.6
86.4
93.8
93.0
83.5
cracking
14.9
7.9
5.9
8.8
8.3
2.8
4.0
18.2
9.0
4.4
5.0
14.5
other, including
0.6
6.2
0.6
1.8
4.5
2.2
2.4
2.2
4.6
1.8
2.0
2.0
39.6
Selectivity (mol %)
dehydrocyclization Product Distribution (mol %) Hydroisomerization 2-MeC6
35.0
41.5
37.2
38.0
39.6
42.1
43.2
39.8
37.5
42.3
42.2
3-MeC6
40.3
42.8
40.1
42.0
42.5
42.4
44.9
39.5
43.4
44.2
43.9
40.7
3-EtC5
3.0
2.5
3.2
3.2
3.1
3.6
3.2
3.3
3.0
3.3
3.2
3.2
2,2-diMeC5
3.4
4.4
3.1
2.9
4.6
1.6
1.4
2.4
5.4
1.2
1.3
2.1
2,3-diMeC5
10.1
4.2
9.0
8.0
5.4
5.5
4.0
8.2
5.3
4.9
5.0
8.0
2,4-diMeC5
6.2
2.9
5.3
3.9
4.0
4.3
2.9
5.5
1.0
3.6
3.7
5.0
3,3-diMeC5
1.4
1.3
1.7
1.7
0.6
0.6
0.4
1.1
0.4
0.5
0.5
1.1
2,2,3-triMeC4
0.7
0.3
0.5
0.3
0.2
0.0
0.1
0.1
3.9
0.0
0.2
0.3
C1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.4
0.0
0.0
0.6
C2
0.1
1.6
0.1
0.0
0.0
1.2
0.0
0.9
2.1
2.3
1.7
0.9
C3
42.9
45.0
44.5
48.5
40.5
47.9
43.0
48.8
39.3
51.4
46.8
50.4
C4
50.4
50.7
53.1
49.8
47.5
46.4
52.5
49.6
48.6
45.4
51.6
47.4
C5
2.8
1.8
1.6
1.3
0.0
4.4
4.5
0.7
5.7
0.9
0.0
0.6
C6
3.9
0.9
0.7
0.4
11.9
0.0
0.0
0.0
3.0
0.0
0.0
0.0
RON I/C
15.2 5.9
14.0 22.0
16.9 17.8
15.9 12.6
8.2 22.3
6.6 34.2
7.6 57.3
6.1 5.0
7.7 9.6
7.4 37.3
9.4 30.6
7.7 6.7
Cracking
mono/multi
3.6
6.6
4.1
4.9
5.8
7.4
10.4
4.8
5.2
8.7
8.4
5.1
i-C4/C4
23.6
16.0
31.0
7.6
128.4
17.6
13.6
11.9
17.4
18.6
24.2
11.1
concentration inside the pores at 8 bar. CBV-760 has the least porosity and lowest average pore diameter among the four samples, and hence increased pressure may then overcome any diffusion limitation at 1 bar. Further increase in pressure to 15 bar resulted in an expected decrease in cracking activity for CBV-760. The high yield of C6 products with USY-B compared to the other catalysts, given the almost complete absence of methane in its products, suggests that some oligomerizationcracking (also called dimerizationcracking) takes place on this catalyst, since the only way for C6 to be produced without producing C1 is for heptane to oligomerize to C14, which then cracks to C6 and C8, with the latter further cracking to C5 and C3 or to two C4's. The BET characterization supports this assumption with this catalyst having the largest porosity among the four catalysts, which would allow for heptane to oligomerize inside the zeolite. This high porosity, however, comes in conjunction with a low average pore diameter, which means that, once the bulky C14 molecule forms, it would have reduced diffusivity in the zeolite structure and hence increased cracking. Over the pressure range, CBV-712 tended to produce the highest blended RON with USY-C producing the lowest. Despite the high isomer yield of USY-C, mostly monobranched isomers are produced. This behavior could be due to the generation of mesopores on steaming and hence the low crystallinity and surface area compared to the other
catalysts. This low crystallinity and surface area would result in reduced transformation to multibranched isomers. Increased pressure also resulted in successive increases in the yield of multibranched isomers.
’ CONCLUSIONS The study of PtUSY zeolite catalysts in the hydroisomerization of normal alkanes is of great interest. In-house catalysts that were generated by steaming zeolite Y at different severities behaved very differently from each other and from acid-leached commercial catalysts. All the steamed catalysts produced higher coke yields than the acid-leached catalysts. Mild steaming produced a catalyst that deactivated rapidly due to high Brønsted acidity and small pores. Medium-strength steaming produced a stable catalyst that generated more cracking and cyclic products. Severe steaming generated a catalyst (USY-C) that, although losing some crystallinity on testing, operated in a near-ideal bifunctional fashion. At high pressures, USY-C produced isomers with a RON reaching 30, a substantial improvement given that the feed n-heptane had an RON = 0. EFAL species appear to contribute to coke generation, especially when present at high concentrations. A mild acid-leaching treatment can therefore greatly enhance catalyst performance. 9923
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Industrial & Engineering Chemistry Research However, too great a dealumination (CBV-760) causes activity to drop sharply and hence require increased temperatures to achieve high RONs. A well-balanced acidmetal function results in the highest RON for product isomers, as demonstrated by CBV-712. CBV-712 probably has an optimum ratio of Brønsted to Lewis acid sites due to its “clean” pore structure. One drawback when using PtUSY zeolites is their high sensitivity to sulfur, requiring a hydrotreated feed.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors would like to thank Dr. Aaron Akah from The University of Manchester for his kind help in carrying out the TGA analysis for spent samples. ’ REFERENCES (1) Eswaramoorthi, I.; Lingappan, N. Ni-Pt/H-Y Zeolite Catalysts for Hydroisomerization of n-hexane and n-heptane. Catal. Lett. 2003, 87 (34), 133–142. (2) Sakagami, H.; Ohno, T.; Takahashi, N.; Matsuda, T. The Effects of Na Loading on Catalytic Properties of H2-reduced Pt/MoO3 for Heptane Isomerization. J. Catal. 2006, 241, 296–303. (3) Liu, Y.; Guo, W.; Zhao, X. S.; Lian, J.; Dou, J.; Kooli, F. Zeolite Beta Catalysts for n-C7 Hydroisomerization. J. Porous Mater. 2006, 13, 359–364. (4) Arribas, M. A.; Marquez, F.; Martinez, A. Activity, Selectivity, and Sulfur Resistance of Pt/WOx-ZrO2 and Pt/Beta Catalysts for the Simultaneous Hydroisomerization of n-Heptane and Hydrogenation of Benzene. J. Catal. 2000, 190, 309–319. (5) Khurshid, M.; Al-Khattaf, S. S. n-Heptane Isomerization over Pt/WO3-ZrO2: A Kinetic Study. Appl. Catal., A: Gen. 2009, 368, 56–64. (6) Akhmedov, V. M.; Al-Khowaiter, S. H. Recent Advances and Future Aspects in the Selective Isomerization of High n-Alkanes. Catal. Rev. 2007, 49, 33–139. (7) Barsi, F. V.; Cardoso, D. Bimetallic Pt-Ni Catalysts Supported on USY Zeolite for n-Hexane Isomerization. Braz. J. Chem. Eng. 2009, 262, 353–360. (8) Ali, L. I.; Ali, A. A.; Aboul-Fotouh, S. M.; Aboul-Gheit, A. K. Hydroconversion of n-paraffins in Light Naphtha Using Pt/Al2O3 Catalysts Promoted with Noble Metals and/or Chlorine. Appl. Catal., A: Gen. 2001, 205, 129–146. (9) Karthikeyan, D.; Lingappan, N.; Sivasankar, B. Hydroisomerization of n-Octane over Bifunctional Ni-Pd/HY Zeolite Catalysts. Ind. Eng. Chem. Res. 2008, 47, 6538–6546. (10) Pope, T. D.; Kriz, J. F.; Stanciulescu, M.; Monnier, J. A Study of Catalyst Formulations for Isomerisation of C7 Hydrocarbons. Appl. Catal., A: Gen. 2002, 233, 45–62. (11) Ramos, M. J.; Gomez, J. P.; Dorado, F.; Sanchez, P.; Valverde, J. L. Hydroisomerization of a Refinery Naphtha Stream over Platinum Zeolite-based Catalysts. Chem. Eng. J. 2007, 126, 13–21. (12) Raybaud, P.; Patrigeon, A.; Toulhoat, H. The Origin of the C7-Hydroconversion Selectivities on Y, β, ZSM-22, ZSM-23, and EU-1 Zeolites. J. Catal. 2001, 197, 98–112. (13) Maloncy, M. L.; Maschmeyer, T.; Jansen, J. C. Technical and Economical Evaluation of a Zeolite Membrane Based Heptane Hydroisomerization Process. Chem. Eng. J. 2005, 106, 187–195. (14) Matsuda, T.; Watanabe, K.; Sakagami, H.; Takahashi, N. Catalytic Properties of H2-reduced MoO3 and Pt/zeolites for the Isomerization of Pentane, Hexane, and Heptane. Appl. Catal., A: Gen. 2003, 242, 267–274.
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