Energy & Fuels 2002, 16, 855-863
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Mild Hydrocracking of Synthetic Crude Gas Oil over Pt Supported on Pillared and Delaminated Clays Hong Yang,* Michael Wilson, Craig Fairbridge, and Zbigniew Ring National Centre for Upgrading Technology, Devon, AB, Canada, T9G 1A8
Josephine M. Hill Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada, T2N 1N4 Received February 15, 2002
A comparison of the mild hydrocracking activities and selectivities to ring opening of Pt supported on pillared and delaminated clays is presented in this study. The catalysts were prepared as extrudates with 30% alumina and were used to process a gas oil fraction derived from Canadian oil sands that was first hydrotreated to reduce the heteroatom content. Experiments were performed in both hydrogenation and mild hydrocracking regimes using a fixed-bed microreactor between 260 and 380 °C, at a total pressure of 10.3 MPa and a LHSV of 1.0. Feed and total liquid products were characterized by using ASTM standard methods and by GC-MS for compositional analysis. Results show that under similar reaction conditions, Pt-supported pillared clay had higher hydrogenation and hydrocracking activities than Ptsupported delaminated clay. However, the latter catalyst had a higher selectivity to diesel yield and produced less naphtha for the same conversion of the +343 °C fraction. Detailed compositional analysis of total liquid products over the operating temperature range also indicated that the Pt/pillared clay catalyst achieved ring opening of two- and three-ring naphthenes, while the Pt/ delaminated clay converted only three-ring species. Catalyst deactivation was evaluated by monitoring feedstock conversion and by examination of fresh and used catalysts after approximately 300 h time-on-stream. The surface area and pore size distributions measured by N2-adsorption/desorption of fresh and used catalysts were compared to determine loss of microporosity due to deposition of carbonaceous materials and possible changes to the catalyst structure due to the reaction environment. Results suggest that pores with diameters less than 60 Å were more susceptible to coke formation.
Introduction Aromatics hydrogenation and mild hydrocracking are important catalytic processes for the production of highquality diesel fuels from petroleum feedstocks. Aromatics, and in particular polyaromatics, contribute to particulate emissions from diesel engines. Polyaromatics are linked to NOx emissions from diesel exhaust gases by affecting ignition temperature.1,2 When mono- and polyaromatics are fully hydrogenated, a significant increase in cetane number results. Further improvement in cetane number can be achieved by opening the saturated rings through a mild hydrocracking process. Mild hydrocracking has advantages over conventional gas oil hydrocracking in minimizing the production of undesirable light hydrocarbons and reducing hydrogen consumption. The ability of certain types of layered clays, i.e., smectites, to swell and expand in one dimension through * Corresponding author. E-mail:
[email protected]. Fax: 1-780987-5349. (1) Diesel Fuel Specifications and Demand for the 21st Century. UOP publication, 1999. (2) Mitchell, K. SAE Paper Technical Paper Series; Society of Automotive Engineers: Warrendale, PA, 2000; Paper No. 2890.
a process of intercalation, has provided a number of potential applications in petroleum refining.3-12 Pillared interlayered clays (PILCs) are synthesized by ionexchange of Na+ or Ca2+ cations within the clay sheets with large polyoxocations of Al, Zr, or Ti. The resultant materials after calcination contain oxide pillars that prop open the sheets and thus expose the internal surface of the clay layers. PILCs have a special geometry (3) Pinnavaia, T. J.; Tzou, M. S.; Landau, S. D.; Raythatha, R. H. J. Mol. Catal. 1984, 27, 195-212. (4) Occelli, M. L.; Landau, S. D.; Pinnavaia, T. J. J. Catal. 1984, 90, 256-260. (5) Occelli, M. L.; Landau, S. D.; Pinnavaia, T. J. J. Catal. 1987, 104, 331-338. (6) Occelli, M. L.; Rennard, R. J. Catal. Today. 1988, 2, 309-320. (7) He Ming-Yuan; Liu Zhonghui; Min Enze. Catal. Today. 1988, 2, 321-338. (8) Occelli, M. L. Catal. Today. 1988, 2, 339-356. (9) Figueras, F. Catal. Rev. Sci. Eng. 1988, 30, 457-499. (10) Wilson, M. F.; Charland, J. P.; Yamaguchi, E.; Suzuki. T. In Hydrotreating Technology for Pollution Control; Occelli, M. L., Chianelli, R., Eds.; Marcel Dekker: New York, 1996; pp 291-311. (11) Kimbara, N.; Charland, J. P.; Wilson, M. F. Ind. Eng. Chem. Res. 1996, 35, 3874-3883. (12) Monnier, J.; Charland, J. P.; Brown, J. R.; Wilson, M. F. In New Frontiers in Catalysis, Proceeding of the 10th International Congress on Catalysis, 19-24 July, 1992, Budapest, Guezl et al., Eds.; Elsevier: Amsterdam, 1993; pp 1943-1946.
10.1021/ef020014l CCC: $22.00 © 2002 American Chemical Society Published on Web 06/18/2002
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that is most suitable for processing ring compounds. In particular, because of their two-dimensional structure, aromatic compounds can intercalate between the sheets of the layered materials and undergo hydrogenation and cracking reactions. The pillars formed within the interlamellar space also provide mainly Lewis acidity, which is a less active cracking component than the Brønsted acidity found in zeolites. The modified acidity in PILC materials is conducive to catalyzing mild hydrocracking reactions. Earlier work concerned with hydrocracking vacuum gas oil over pillared bentonites was carried out by Occelli and Rennard.6 They used a sulfided Ni-Mo catalyst ingredient as a hydrogenating component, which is markedly less active than the noble metal used in this study. Work carried out previously in our laboratories compared the performance of Ni-Mosupported on pillared clay with conventional Y-zeolite for hydrocracking synthetic crude gas oil.12 The catalyst containing pillared clay was found to be superior in utilizing hydrogen more efficiently and producing higher liquid yields with considerably lower amounts of light gases at the same gas oil conversion. The differences in catalyst performance were attributed to the more open pore structure of the PILC material and possible aggregation of clay lamellas, which are both expected to enhance diffusion of multi-ring gas oil components. Significantly higher operating temperatures are needed for hydrogenation of aromatics in gas oils over sulfided Ni-Mo. Overcracking, which may be encountered at higher reaction temperatures, leads to the production of undesirable secondary products. In addition, higher temperatures shift the thermodynamic equilibrium in favor of aromatics formation and, therefore, work against the ring opening process. To counter the equilibrium shift, higher hydrogen partial pressures are required resulting in more expensive refinery operations. An objective of the present work was to obtain greater selectivity in the hydrocracking process by using more active platinum ion-exchanged clay materials. Lower reaction temperatures are used, which should allow greater control of the hydrocracking process and improve selectivity in ring opening reactions. Aromatics dehydrogenation through shift in the thermodynamic equilibrium reaction at higher temperatures is also avoided. Occelli and Rennard also examined the effect of using different pillaring agents in their experiments and found that a range of cracking activity in gas oil was obtained.6 They showed that the most active cracking material in pillared clays was the aluminum chlorohydrate (ACH) pillared bentonite. However, they also noted that for pillared clays in general, the Lewis/ Brønsted (L/B) acid site ratio is typically between 4 and 8 (at 400 °C in vacuo), whereas in HY zeolite the estimated ratio was 0.6.6 In this work we chose the most active pillaring agent, ACH (the [Al13O4(OH)24 (H2O)12]7+ cation), which forms alumina pillars after ion-exchange with montmorillonite and calcination at 400 °C. Delaminated clays have disordered edge-to-edge and face-to-edge aggregations (house-of-cards type structures) when compared with the ordered stacking arrangement or face-to-face aggregations in PILCs. In addition to micropores of the type found in PILCs, delaminated clays also contain mesopores and macro-
Yang et al.
pores that are formed from face-to-edge aggregations, which favor diffusion and accommodation of higher molecular-weight hydrocarbons of the type contained in middle distillates derived from Canadian oil sands.3-5,8,10 However, delaminated clays have not been used previously for hydrocracking gas oils. Occelli reviewed and compared the performance of delaminated clays with those of pillared clays for fluid catalytic cracking of a gas oil using a standard microactivty test (MAT).8 His experiments involved a study of the surface properties and cracking activity of the clay materials. Occelli found that delaminated clays are somewhat less active than pillared clays as cracking catalysts, but exhibit greater selectivity for gasoline production and a lower coke yield. He attributed the improved coke selectivity to the weaker acidity of the delaminated material and the presence of macroporosity, which he claimed would favor the desorption of high molecular weight hydrocarbons that would otherwise be retained as coke. Previous work done in our group evaluated the performance of a Ni-Mo catalyst supported on a delaminated clay for hydrotreating coal-derived liquids.10 The primary objectives were to hydrogenate the feedstock, remove heteroatoms, and saturate aromatics. In support of Occelli’s findings, catalyst deactivation by adsorption of various foulants was reduced significantly using the delaminated clay as catalyst support.10 Bifunctional catalysts, having a hydrogenation component and a shape-selective cracking component, present good prospects for ring opening.13 The catalysts employed contain a noble metal, in this case platinum, supported on a molecular sieve material. As part of the catalyst development strategy, it was the intention of the present study to prepare support materials with optimum pore structures to accommodate the molecular size range found in synthetic crude gas oils. It is important that the support contain a high percentage of accessible pores, which should also contain a suitable acidic cracking function. In this work the mild hydrocracking activities of two different types of platinumsupported clay materials were compared. The materials were a montmorillonite and a synthetic hectorites Laponite RDSsthat were used to study the effects of catalyst pore structure, surface area and surface acidity on the processing of heavy gas oil hydrocarbons. Montmorillonite has a pancakelike morphology and a layer lateral dimension in the range 102-104Å. Thus, it has a larger face-to-face/edge-to-face layer association ratio and tends to aggregate in a face-to-face lamellar configuration. Accordingly, montmorillonites are more readily pillared than delaminated. In contrast, in diluted aqueous dispersions, Laponite particles exist as discrete platelike crystals with diameters of 200-300 Å and a thickness within the range 1-40 Å.14 The particle size and lath-like morphology of Laponite provides conditions for it to produce a delaminated structure, when reacted with a pillaring agent in aqueous dispersions. Experimental Section Catalyst Preparation. Pt-supported pillared clay (Pt/ PILC) catalyst was prepared according to published procedures (13) Weitkamp, J.; Ernst, S.; Karge, H. G. Erdol Kohle 1984, 37, 457-462. (14) Thompson, D. W.; Butterworth: J. T. J. Colloid Interface Sci. 1992, 151, 236-243.
Hydrocracking of Synthetic Crude Gas Oil using Accofloc 350, a naturally occurring montmorillonite supplied by the American Colloid Company.11 The starting material for preparing Pt-supported delaminated clay (Pt/DILC) catalyst was a synthetic hectorite, Laponite RDS, supplied by Southern Clay Products Inc. (Gonzales, TX). Laponite RDS was added slowly to deionized water to make a suspension of 3.7 wt %. After the suspension was stirred for 30 min to allow it to fully hydrate, the temperature was raised to 50 °C and a calculated amount of pre-refluxed 50% Chlorohydrol pillaring agent (Reheis, Berkeley Heights, NJ) was added dropwise with constant stirring. The amount of Chlorohydrol solution added was a 10-fold excess over that required from the cation exchange equivalent of Laponite RDS. After 16 h of reaction the suspension was centrifuged and washed repeatedly until chloride ion in the washings was in the 10-4 M concentration range. The delaminated material was first freeze-dried and then oven-dried at 110 °C for 4 h, and finally calcined at 400 °C for 4 h. The calcined material was treated in suspension with 6% ammonia to regenerate the cation exchange capacity. The ammonia-treated material was washed and centrifuged free of ammonium ion, dried in air, and ovendried at 110 °C. Platinum was loaded by making a suspension of 50 g of delaminated Laponite RDS (20 Å) (m2/g)
pore volume (>20 Å) (cm3/g)
Si/Al ratio wt %
Pt wt %
Pt/PILC Pt/PILC-alumina fresh Pt/PILC -alumina used Pt/DILC Pt/DILC-alumina fresh Pt/DILC -alumina used
130 160 74 256 284 87
0.105 0.158 0.097 0.150 0.234 0.129
67 40 3 79 50 6
0.031 0.019 0.001 0.035 0.021 0.002
63 120 71 177 234 81
0.074 0.140 0.097 0.115 0.213 0.127
1.86
1.44
1.78
1.03
mm2/s, 40 °C) were determined using ASTM D4052, D611, and D445, respectively Surface areas and pore size distributions of fresh and used catalysts were determined using a Micromeritics ASAP 2010C unit. Total surface areas were obtained using the BET N2 adsorption method. The used catalyst extrudates were Soxhlet extracted overnight with tetrahydrofuran, before being subjected to N2 adsorption/desorption analysis. The t-plot method was used to calculate micropore areas and volumes, and the total pore areas and volumes for pores > 20 Å were calculated by difference. Density functional theory (DFT) software supplied by Micromertics was used to calculate pore volume distributions. The density and strength of the acid sites of ammoniatreated pillared clay and ammonia-treated delaminated clay were determined by ammonia temperature programmed desorption (NH3-TPD). A 0.2 g sample was placed in a quartz flow cell and attached to a gas handling and vacuum system (Advance Scientific Designs Inc.). The sample was first heated to 500 °C over 1 h in flowing helium (60 mL/min), and held at this temperature for 2 h before it was cooled to 100 °C and exposed to a stream of ammonia in helium (9.79% NH3, Praxair) for 1 h. The sample was then purged with helium for 20 min. During TPD, the sample was heated at 10 °C/min to 600 °C in 25 mL/min (STP) He, and held at 600 °C for up to 20 min. A fraction of the gases exiting the sample cell was directed to a quadrapole mass spectrometer (UTI 100C) through a leak valve. The pressure in the mass spectrometer was maintained at 2.0 × 10-4 Pa, and calibration of the mass 17 signal was performed at the beginning of each experiment using a standard gas mixture of NH3 in He. The contents of Pt, Al, and Si in Pt/PILC and Pt/DILC materials were measured by inductively coupled plasma-mass spectrometry (ICP-MS) at the Alberta Research Council. The catalyst samples were first acid digested under microwave heating using a QWAVE-1000 microwave sample preparation system (Questron, Mercerville, NJ), equipped with temperature and pressure regulation. The ICP-MS system used for analysis was a Perkin-Elmer Elan 5000 ICP quadrupole mass spectrometer (Thornhill, ON, Canada), equipped with a GemTip cross-flow nebulizer, Ryton spray chamber, plasma torch with a quartz injector, a Gilson four-channel peristaltic pump (Model Minpuls III), and a Gilson 212B autosampler. A detailed analytical procedure can be found elsewhere.23
Results and Discussion Characterization of Catalyst Materials. The BET surface area, micropore area and micropore volume of (20) Allard, L. N.; Webster, G. D.; Ryan, T. W.; Baker, G., Beregszaszy, A.; Fairbridge C. W.; Ecker, A.; Rath, J. SAE Technical Paper Series; Society of Automotive Engineers: Warrendale, PA, 1999. Paper No. 3591. (21) Steere, D. E. SAE Technical Paper Series; In Procedings of the Fuels and Lubricants Meeting, Baltimore, Maryland, 8-11 October, 1984; Society of Automotive Engineers: Warrendale, PA; Paper No. 841344, 1984. (22) Pulley, R. E., Jr.; Daubert, T. E. Division of Refining. In Technical Data Book-Petroleum Refining, 5th ed.; American Petroleum Institute: Washington, DC, 1992; Chapter 1. (23) Wu, Shaole; Zhao, Yuhui; Xinbang Feng, Xinbang; Wittmeier, A. J. Anal. At. Spectrom 1996, 11, 287-296.
Figure 1. Ammonia TPD curves. (a) Delaminated Laponite. (b) Pillared montmorillonite.
Pt/PILC, Pt/DILC, and catalyst extrudates before and after reaction are summarized in Table 1, together with the contents of Pt, Si, and Al elements for Pt/PILC and Pt/DILC. The results show that Pt/DILC had a significantly higher surface area and pore volume than Pt/ PILC (257 versus 130 m2/g, and 0.150 versus 0.105 cm3/ g). Furthermore, 69% of Pt/DILC surface area was contributed by mesopores and macropores, compared to 48% in the case of Pt/PILC. Pt/DILC also had a higher percentage of pore volume with pores greater than 20 Å (77%), than Pt/PILC (70%). These results are consistent with the structural differences found between pillared and delaminated clays. As shown in previous work from our laboratories, and from other studies, delaminated clays using Laponite-based materials have a more open pore structure with a well-defined mesoporosity.3,5,8,10 Hence, the data provide evidence that by adding a pillaring agent to a dilute aqueous suspension of Laponite RDS, a delaminated clay structure with a combination of micropores and mesopores is produced after suitable drying. Table 1 also shows that the final catalyst extrudates had higher BET surface areas and total pore volumes due to the addition of alumina. Results of the acidity measurements (NH3-TPD) of the ammonia-treated pillared montmorillonite and delaminated Laponite RDS materials are presented in Figure 1. Essentially, a single maximum is observed for both catalysts, which is a broad peak covering the temperature range from 120 to 500 °C, with a series of visible shoulders. This broad peak was reported by several scientific papers.21,24-26 By using a curve deconvo(24) Arena, F.; Dario, R.; Parmalina A Appl. Catal A 1998, 170, 127137. (25) Gil, A.; Vicente, M. A.; Gandia, L. M. Catal. Today. 2001, 68, 41-51. (26) Iwamatsu, E.; Hayashi, E.; Sanada, Y.; Ahmed, S.; Ahmed, Ali S.; Lee A. K. K.; Hamid, H.; Yoneda, T. Appl. Catal. A 1999, 179, 139144.
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Table 2. Physical and Chemical Properties of the Feed and Total Liquid Products (Hydrogen Pressure 10.3 MPa, LHSV 1.0, Hydrogen Flow 600NL/L Feed) reaction temperatures (°C) Pt/PILC-alumina wt % of fractions IBP-177 °C (naphtha) 177-343 °C (diesel) +343 °C conversion of +343 °C % density (g/mL) H/C ratio viscocity (40 °C) (cSt, mm2 s-1) aniline point (°C) cetane IQT cetane D613 CI(CGSB)
feed 3.3 65.8 30.8 0.8628 1.85 5.49 75.3 51.4 50.4 54.0
Pt/DILC-alumina
260
280
300
320
300
320
340
360
380
5.5 66.0 28.5 7.6 0.8527 1.93 4.90
8.2 65.7 26.1 15.4 0.8460 1.89 4.21
17.7 63.7 18.6 39.8 0.8274 1.93 2.61
30.2 58.6 11.2 63.8 0.8052 1.99 1.75
3.8 66.6 29.6 4.0 0.8539 1.87 5.37
4.3 66.9 28.9 6.3 0.8519 1.90 5.12
6.5 67.3 26.2 15.1 0.8464 1.89 4.51
10.5 67.1 22.3 27.7 0.8361 1.95 3.50
12.3 66.8 20.9 32.2 0.8325 1.89 3.26
80 51.4
81.6 52.5
77.3 48.8
73.7
83.0 53.6
82.8 54.1
82.4 53.4
80 53.3
79.5 53.2
58.2
59.7
56.8
54.6
60.6
60.6
60.5
58.9
58.5
lution method, Arena et al. showed that three maxima at respective temperatures 232, 301, and 405 °C, contributed to this peak for a K10 montmorillonite.21 These maxima corresponded to three types of acid sites (weak, medium and strong), from which NH3 was released with no discrimination between Lewis and Bro¨nsted acidity on the basis of NH3-TPD results. The quantity of acid sites contained in peak b, as shown in Figure 1 for pillared montmorillonite, are significantly lower than those of the delaminated Laponite (peak a). By comparison of the compositions of the parent clays {montmorillonite:(0.5Ca,Na)0.66 Si8Al3.34 Mg0.66; Laponite: (0.5Ca,Na)0.66 Si8Mg5.34Li0.66} with those of the pillared montmorillonite and delaminated Laponite (Table 1), it can be seen that the Si/Al ratio of Laponite decreased from infinity to 1.78 when it was reacted with the pillaring agent, which corresponds to 4.49 Al atoms for every 8 Si atoms. However, the Si/Al ratio of montmorillonite was only changed from 2.40 in the parent clay to 1.86 in the pillared clay, which corresponds to 4.30 Al atoms per 8 Si atoms, an increase of 0.96 from the parent clay. These results indicate that delaminated Laponite has more pillars inserted between its sheets than pillared montmorillonite, since all of its Al atoms are assumed to be contained within the pillars. The higher density of pillars in delaminated Laponite is responsible for the large amount of NH3 released, which is consistent with the results reported in the literature.7,9 The rise of the NH3 peak at temperatures around 600 °C was checked by another TPD experiment, which was run under the same condition as the previous ones without exposing the sample to NH3. It was found that water was responsible for this high-temperature peak for both clay-based catalyst precursors. Mild Hydrocracking Activities of Experimental Catalysts. Catalyst activities were evaluated by carrying out mild hydrocracking experiments at 260-320 °C for Pt/PILC-alumina and at 300-380 °C for Pt/DILCalumina. Table 2 presents physical and chemical properties of feedstock and total liquid products derived from the experiments carried out under different reaction conditions. It is apparent that Pt/PILC-alumina was active in the lower temperature range and had a significantly higher hydrocracking activity than Pt/ DILC-alumina. The difference in activity is shown notably by the results obtained for both catalysts at 320 °C. At this temperature, conversion of the 343 °C+ fraction was 63.8% for Pt/PILC-alumina compared to
6.3% for Pt/DILC-alumina. However, under these conditions Pt/PILC-alumina produced a significantly higher naphtha yield (30.2%) at the expense of the diesel yield. Although the delaminated clay material has a larger amount of acid sites, the results from our hydrocracking experiments indicate that it has a lower cracking activity than pillared clay. Using pyridine/FTIR adsorption studies, Occelli has claimed that the strength of acid sites in delaminated Laponite are weaker than those found in pillared clay. He attributed the weaker acidity and reduced cracking activity of the delaminated clay to differences in acid site availability resulting from stacking disorders.8 In this work, Figure 1 reveals that ammonia desorption occurs in the same temperature range for both catalysts and the acid strength appears the same. Hence, the results obtained in Occelli’s study appear to contradict our work. However, we conclude that the discrepancy is due to the difference in molecular size and diffusion rates for pyridine and NH3. In the NH3-TPD experiments, the smaller ammonia molecules were able to diffuse more easily through the threedimensional structure of the delaminated clay and, hence, more accessible acid sites were detected. Accordingly, in the hydrogenation and hydrocracking study, we conclude that gas oil molecules diffusing into the delaminated clay structure are more exposed to the exterior clay surfaces arranged in a random house-ofcards orientation, and the pillars containing the Lewis acid sites are less accessible. Figure 2 shows plots of diesel and naphtha yields (wt %) versus percent conversion of the +343 °C fraction from mild hydrocracking experiments over both catalysts. Pt/DILC-alumina gave higher diesel and lower naphtha yields. The selectivity of the delaminated clay catalyst toward higher diesel and lower naphtha yields may be related to the lower amount of accessible acid sites in the support material, which should moderate the cracking activity.4,5,7,8 It is also likely that overcracking of the heavy feedstock material is alleviated by the house of cards structure of the Pt/DILC-alumina, which should facilitate desorption of the products from the catalyst surface. Furthermore, because Laponite RDS is a synthetic clay, it does not contain the iron impurities that exist in Accofloc 350, the naturally occurring montmorillonite used in this study. Other researchers have claimed that iron can promote cracking of heavier hydrocarbons to lighter fractions, and the
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Figure 2. Naphtha and diesel yields as function of the conversion of +343 °C. (O) Pt/DILC-alumina, diesel; (0) Pt/ DILC-alumina, naphtha; (b) Pt/PILC-alumina, diesel; (9) Pt/ PILC-alumina, naphtha.
selectivity toward diesel yield may be affected accordingly in reactions over the Pt/PILC-alumina catalyst.4,8 Table 2 also shows that a marked improvement in the atomic hydrogen to carbon (H/C) ratios was achieved in products generated over both catalysts. However, it is worth noting that the experiments at lower reaction temperatures using the Pt/PILC-alumina catalyst gave products with higher hydrogen contents, which demonstrates higher hydrogenation activity. For the Pt/DILCalumina catalyst, the H/C ratio of the total liquid product reached a maximum at 360 °C, i.e., 1.95, and then declined. The data suggest that at the maximum operating temperature, 380 °C, there was a shift in the thermodynamic equilibrium reaction favoring abstraction of hydrogen from saturated ring compounds and aromatics formation. Impact of the Reaction Conditions on Diesel Ignition Quality. Cetane number by ASTM D613 is a measure of the ignition quality of a diesel fuel in a standard compression-ignition engine. While ignition delay can be influenced by engine type and operating conditions, cetane number is a function of the chemical composition of the fuel. In general, normal paraffins have the strongest positive effect upon cetane number followed by isoparaffins, moncycloparaffins, alkylbenzenes, polycycloparaffins, and polyaromatics.27-29 Cetane number is one of the most important diesel specifications and has been targeted for environmental regulation along with density, sulfur content, distillation temperature, and aromatics content. Studies show that increasing cetane number improves fuel combustion, reduces NOx and, to a minor extent, reduces engine particulate emissions.2,30-32 Cetane indices represent various attempts to predict cetane numbers by means (27) Ju, T. Y.; Wood, C. E. Oil Gas J. 1941, 9, 41, 43-44, 46. (28) Olson, D. R.; Meckel, N. T.; Quillian, R. D., Jr. SAE Technical Paper Series; Society of Automotive Engineers: New York, 1960; Paper No. 263 A. (29) Heck, S. M.; Pritchard, H. O.; Griffiths, J. F. J. Chem. Soc., Faraday Trans. 1998, 94, 1725-1727. (30) Naber, D.; Lange, W. W.; Reglitzky, A. A.; Schater, A.; Gairing, M.; Le’Jeune, A. SAE Paper No. 932685; Society of Automotive Engineers: Warrendale, PA, 1993. (31) McCarty, C. I.; Slodowske, W. J.; Sienicki, E. J.; Jass, R. E. Fuel Reformulation 1994, 3-4, 34. (32) Diesel fuel technical review. Chevron Products Company: San Francisco, 1998.
Yang et al.
of correlations and test methods other than the ASTM D613 engine test. This work shows that the Pt/DILCalumina catalyst gave products with higher ignition quality than Pt/PILC-alumina at the same conversion levels as predicted by both the IQT test and CGSB method. For both catalysts the index also reaches a maximum with increased reaction severity and then declines, due to overcracking and, probably, shifts in thermodynamic equilibrium at higher temperatures. Compositional Analysis of Liquid Products. Previous work concerning aromatics hydrogenation and mild hydrocracking of a hydrotreated blend of Syncrude Canada Ltd. light gas oils was reported by Wu, Reno, and Brierley.33 These workers used zeolite-supported noble metal catalyst, which could be operated in either an aromatics hydrogenation regime or a hydrocracking regime, to process the light gas oil material. Under the hydrocracking regime, cetane numbers in the range 4549 were achieved by operating in the approximate temperature range 330°-370 °C. Under these conditions, the authors claimed that both aromatics hydrogenation and cracking reactions generated paraffins and reduced the concentration of ring structures. However, their results showed that in the hydrocracking regime, the production of paraffins was low. Mild hydrocracking of cycloparaffins produced a 5 wt % increase in single ring naphthenes and a corresponding increase in naphtha yield because of formation of lighter products boiling outside of the diesel range.33 In the present work, four saturated hydrocarbon group types (paraffins, monocycloparaffins, dicycloparaffins, and tricycloparaffins) and three aromatic group types (monoaromatics, diaromatics, and triaromatics) were effectively identified by the GC-MS method employed. Detailed chemical compositional analyses of the feedstock and total liquid products at different reaction temperatures are presented in Table 3. The corresponding plots are presented in Figures 3 and 4 for Pt/PILCalumina and Pt/DILC-alumina catalysts, respectively. Pt/PILC-alumina was much more active for conversion of the feedstock components in the lower temperature range and the overall production of paraffins and monocycloparaffins was significantly greater than for the Pt/DILC-alumina. Paraffins may be formed by cracking of side chains and ring opening of monocycloparaffins, and monocycloparaffins are derived from saturation of monoaromatics, ring scission in dicycloparaffins, or center ring cracking of saturated 3-ring structures. Figure 4 shows that the hydrocracking activity of the Pt/DILC-alumina catalyst was much more controlled, and it is significant that the formation of dicycloparaffins over this catalyst was sustained over the entire temperature range, indicating resistance to cracking. Greater amounts of dicycloparaffins were also produced, unlike those produced for Pt/PILC-alumina, which reached a maximum at 300 °C and then declined. For the Pt/DILC-alumina there was also a steady increase in the amounts of monocycloparaffins, and these results suggest that for this catalyst the conversion of tricycloparaffins was more selective, and ring opening occurred with greater control of the cracking process, which mitigated overcracking to lighter prod(33) Wu, H. H.; Reno, M. E.; Brierley, G. R. AIChE Spring National Meeting, New Orleans, Louisiana, February, 1996; Paper 57C,
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Table 3. Chemical Composition (mass %) of the Feed and Total Liquid Products Determined by GC-MS (Hydrogen Pressure 10.3 MPa, LHSV 1.0, Hydrogen Flow 600NL/L Feed) reaction temperatures (°C) Pt/PILC-alumina
Pt/DILC-alumina
hydrocarbon groups
feed
260
280
300
320
300
320
340
360
380
saturates paraffins monocycloparaffins dicycloparaffins tricycloparaffins aromatics monoaromatics diaromatics triaromatics+ conversion of aromatics %
74.6 10.3 22.9 28.2 13.3 25.4 15.9 4.1 3.9
91.3 11.4 29.0 35.3 15.6 8.8 1.7 2.3 4.4 65.5
94.0 11.9 30.9 36.3 14.9 6.1 0.5 1.9 3.7 76.1
95.6 13.9 34.1 35.8 11.9 4.5 0.3 1.0 3.2 82.4
97.6 16.7 38.0 34.0 8.9 2.5 0.3 0.2 2.2 90.2
93.2 11.4 28.5 35.2 18.1 6.8 0.9 2.7 3.2 73.1
93.2 11.5 28.8 35.6 17.4 6.8 0.8 2.6 3.3 73.1
93.4 11.8 29.8 36.6 15.2 6.6 0.8 2.4 3.3 73.9
95.1 12.5 32.0 38.3 12.3 4.9 0.8 1.2 2.8 80.6
96.0 12.9 32.9 38.8 11.4 4.0 1.1 0.8 2.1 84.2
Figure 3. Mass percent of saturates and total aromatics versus reaction temperature over Pt/PILC catalyst. (b) Paraffins. (O) Monocycloparaffins. (9) Dicycloparaffins. (0) Tricycloparaffins. ([) Total aromatics.
Figure 4. Mass percent of saturates and total aromatics versus reaction temperature over Pt/DILC catalyst. (b) Paraffins. (O) Monocycloparaffins. (9) Dicycloparaffins. (0) Tricycloparaffins. ([) Total aromatics.
ucts. The implication is that both center ring and single ring scission occurred with greater selectivity and the results more closely meet the objectives of the mild hydrocracking process. As shown in Figures 3 and 4, the concentrations of tricycloparaffins steadily declined in the temperature ranges studied. These results suggest that the pore structures of both Pt/DILC-alumina and Pt/PILC-alumian are suitable for ring opening reactions. For aromatics conversion the data show that
Figure 5. Distribution of paraffins in different boiling ranges for Pt/PILC-alumina catalyst.
Pt/PILC-alumina was a more active catalyst, giving an 88.9% conversion at 320 °C compared with a maximum of 83.4% for the Pt/DILC-alumina catalyst at 380 °C. The higher Pt content of Pt/PILC (Table 1) might be responsible for its superior aromatic saturation activity. Another reason might be its higher cracking activity, which could convert the naphthenic compounds more effectively to paraffins, thus shifting the aromatics/ naphthenes equilibrium toward the formation of naphthenes. It was noted above that higher yields of paraffins were obtained from reactions over Pt/PILC-alumina in the lower temperature range. To know how the reaction temperature effects the distributions of paraffins, the yields of paraffins in different boiling ranges are presented in Figures 5 and 6, which show changes in paraffins content versus reaction temperature in three fractions: naphtha (IBP-177 °C), diesel (177-343 °C), and heavy distillate (+343 °C). A comparison of these bar charts for the two catalysts indicates that the gain in paraffins for Pt/PILC-alumina was contributed mainly by an increased amount in the naphtha fraction. Thus, over the operating temperature range for this catalyst the amount of paraffins in the diesel fraction increased only marginally and decreased somewhat in the heavy distillate fraction. In contrast, for the Pt/DILC-alumina, there were higher amounts of paraffins generated in the heavy distillate fraction and significantly less in the naphtha fraction, indicating controlled cracking. These results may explain why the Pt/PILC-alumina catalyst gave a higher aromatics conversion without a substan-
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Figure 7. Comparison of the pore volume distributions of the (O) fresh and (b) used Pt/PILC-alumina catalysts. Figure 6. Distribution of paraffins in different boiling ranges for Pt/DILC-alumina catalyst.
tial overall increase in cetane number, and it would appear that the catalyst is not a good choice for improved diesel selectivity. Evaluation of Catalyst Deactivation. During hydroprocessing operations catalyst activity is reduced by physical and/or chemical deactivation processes. In our study, catalyst deactivation was investigated by repeating one of the experiments at the end of the testing period using identical reaction conditions and monitoring conversion of the liquid feedstock. For Pt/PILCalumina, the experiment done at 300 °C was repeated after 290 h of time on stream. By comparing product densities from the duplicate experiments and using the density-temperature relationship in Table 2, it was found that during this experimental program, the catalyst deactivated at an average rate of 0.056 °C/h. For Pt/DILC-alumina, the experiment at 320 °C was repeated after 308 h time on stream, and the corresponding deactivation rate was found to be 0.034 °C/h. This experimental program was not designed to determine catalyst deactivation rates. The period of time on stream was too short and there were only two points to base the estimate on. Therefore, the amount of information is insufficient to provide a reliable estimate of the deactivation rate. However, it is worth noting that similar results were observed by Wilson et al. for a deactivation study involving hydrotreatment of coalderived liquids over Ni-Mo catalysts supported on delaminated Laponite and pillared montmorillonites.10 Surface areas and pore size distributions of fresh and used catalysts measured by N2-adsorption/desorption were compared, to study the influence of reaction environment on catalyst stability. The results in Table 1 show that total surface area decreased significantly for both catalysts after processing the gas oil feedstock. For Pt/PILC-alumina, the surface area of the used catalyst was 45% of the fresh one and for Pt/DILCalumina the corresponding percentage remaining was 31%. The table also shows that both catalysts lost most of their micropore areas and volumes. To elucidate changes in pore size distribution after time on stream, the incremental pore volume distributions of fresh and used catalysts are compared in Figures 7 and 8 for Pt/ PILC-alumina and Pt/DILC-alumina, respectively. A common feature can be observed from the figures. Pores
Figure 8. Comparison of the pore volume distributions of the (O) fresh and used (b) Pt/DILC-alumina catalysts.
with diameters less than 60 Å were most vulnerable to blockage, and since the deposits were resistant to removal by THF Soxhlet extraction, these pores likely were plugged by carbonaceous materials derived from coke precursors contained in the feedstock. In contrast, when the pore diameters are greater than 60 Å, the fresh and used catalysts appear to follow the same pore size distribution curves. The reduction in total surface area and blockage of pores less than 60 Å was more prevalent in the Pt/DILC-alumina catalyst, which might be attributed to the higher reaction temperatures used that favor coke formation. However, since the severity of pore blockage by coke formation on the catalysts is contrary to the deactivation rates given above, the apparent loss of pore area in the used catalysts is not a good indicator of catalyst deactivation. We might conclude this since the catalysts are not characterized under working conditions, i.e., at temperatures and hydrogen pressures where they might continue to function effectively. In his work on the application of delaminated clays for fluid catalytic cracking of heavy gas oils, Occelli determined the dependence of coke make on conversion for several clay-based catalysts, and found a favorable coke selectivity for the delaminated clay catalyst in FCC experiments.8 He attributed the improved result for the delaminated clay to its moderate surface acidity and macroporosity, which he claimed would favor desorption of high molecular weight hydrocarbons that otherwise
Hydrocracking of Synthetic Crude Gas Oil
might be retained as coke. However, to explain the results found here, an alternative argument might apply, i.e., the tendency of the delaminated clay to form coke may be attributed to the more open structure, which allows larger aromatic molecules to diffuse to the surface and readily adsorb in the mesoporous structure. This may be the strongest argument to explain the loss of surface area found here. Occelli has also noted that the interaction of the adsorbed molecules with Lewis acid sites on the dehydroxylated oxocations could facilitate polycondensation of the aromatic centers and lead to coke formation. However, since in this application there is a high hydrogen partial pressure, there should be less tendency for this reaction to occur. Conclusions Pt/PILC-alumina and Pt/DILC-alumina catalysts were used for hydroprocessing a gas oil derived from Canadian oil sands. The activity test results showed that the catalysts had suitable pore structures and surface acidity for mild hydrocracking reactions and produced minimal amounts of light gases. Detailed chemical compositional analyses of feedstock and total liquid products over a range of reaction temperatures showed that Pt/PILC-alumina was able to achieve ring opening in two- and three-ring naphthenes, while Pt/DILCalumina was selective to only three-ring species. Pt/ PILC-alumina was a more active catalyst for conversion of total aromatics and the +343 °C fraction, but was less selective for diesel production and produced excessive amounts of naphtha from secondary cracking reactions. Paraffins produced by hydrocracking were concentrated in the IBP-177 °C fraction, and the diesel
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product consequently had lower cetane numbers as determined by an ignition quality test and CGSB Cetane Index. Pt/DILC-alumina was superior for production of diesel fuel and was more stable, as indicated by catalyst deactivation studies. A comparison of pore size distributions of fresh and used catalysts showed that pores with diameters less than 60 Å were more susceptible to blockage by coke formation. Mesoporous materials having an optimum balance of pore size distribution, surface area, and surface acidity seem to be the best choice for hydrocracking catalyst supports suitable for ring opening reactions. Further work is required to elucidate fundamental differences in acidity between pillared montmorillonites and pillared synthetic hectorites. Acknowledgment. Partial funding for this work has been provided by the Canadian Program for Energy Research and Development (PERD), the Alberta Research Council, and The Alberta Energy Research Institute. The authors gratefully acknowledge Mr. Robert Garez for operating the catalyst testing unit and NCUT analytical laboratory staff for determining the feed and product properties. We thank Dr. R. A. Kydd, Department of Chemistry, University of Calgary, for the TPD measurements. We are also grateful to Mr. David Sporleder, Shell Canada Limited, Calgary Research Centre, for the IQT tests. We thank Syncrude Canada for kindly supplying the LC-Finer LGO. Hong Yang is thankful to the Natural Sciences and Engineering Research Council of Canada for partial financial support. EF020014L
