372
Energy & Fuels 1990,4,372-379
Hydroisomerization and Hydrocracking of n -Heptane and n -Hexadecane on Solid Superacids Michael Y. Wen,* Irving Wender, and John W. Tierney Chemical and Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received February 1, 1990. Revised Manuscript Received April 30, 1990 Formation of highly branched long-chain paraffins and highly isomerized shorter chain paraffins from conversion of heavier n-paraffmic fractions is of interest for the production of high-octane gasoline, jet fuel, and lubricants. In this study, n-hexadecane was hydroisomerized and hydrocracked on a platinum-doped solid superacid, Pt/Zr02/S042-,in a batch horizontal shaking microreactor system at rather mild conditions: 130-170 OC, 300 psig initial hydrogen pressure, and 15-75-min reaction time. A moderate conversion level, over 65 w t % ,was achieved at temperatures above 140 "C, which is significantly lower than Pt-doped acid catalysts in similar studies. Up to 70% of the hydroisomerized products are multibranched isohexadecanes. For the hydrocracked products, a very high selectivity, more than 90%, to isoparaffms resulted. Less than 1wt % of platinum on the surface was very effective in preventing catalyst deactivation under low hydrogen pressure. The presence of small amounts of moisture in the reaction system enhanced catalyst reactivity, probably due to transformation of Lewis acid sites to Brransted acid sites. Product distribution studies indicated that n-hexadecane was first transformed to monobranched isohexadecanes through skeletal isomerization, with consecutive reactions to multibranched isomers. Cracking products were obtained from cleavage of multibranched isohexadecanes; cracking of monobranched isohexadecanes appeared unlikely to occur under the reaction conditions studied.
Introduction Hydrocracking of heavy oil for the production of light fuels, e.g., gasoline, diesel, and jet fuel, is a well-established process that plays a key role in the success of modern refining te~hnology.'-~However, there is great interest in the isomerization of heavy n-paraffinic hydrocarbons leading to the production of jet fuel and/or lubricants. Conventional amorphous bifunctional catalysts, a combination of well-dispersed transition metals on an acidic support, have been used for hydrocracking. A catalyst with a weak hydrogenation function, e.g., sulfided Ni or W, on an acidic support, e.g., A1203 or Si02-A1203,failed to isomerize long-chain n-paraffins to a significant extenta5s6 Selectivities for isomerization of long-chain n-paraffins were improved considerably when platinum, a metal of high hydrogenation function, was used to balance the acidity of the During the past two decades, extensive research on zeolite-based catalysts has resulted in substantial progress not only in hydrocracking but also in hydroisomerization of long-chain n-paraffins. The platinum containing zeolite catalysts'~'' have been shown (1)Choudary, N.; Saraf, D. N. Ind. Eng. Chem. Prod. Res. Deu. 1975, 14,74-83. (2)The Future of Heavy Crude and Tar Sand; Meyer, R. F., Steele, C. T., Eds.; McGraw-Hill: New York, 1979. (3)Corbett, R. A. Oil Gas J. 1989,26,42-46. (4)O'Rear, D. J. Ind. Eng. Chem. Res. 1987,26,2337-2344. (5)Flinn, R. A.; Larson, 0. A.; Beuther, H. Ind. Eng. Chem. 1960,52, 153-156. (6)Archibald, R. C.; Greensfelder, B. A.; Holzman, G.; Rowe, D. H. Ind. Eng. Chem. 1960,52,745-750. (7)Gibson, J. W.; Good, G. M.; Holzman, G. Ind. Eng. Chem. 1960, 52, 113-116. (8)Coonradt, H. L.; Garwood, W. E. Ind. Eng. Chem. Proc. Des. Deu. 1964. 3.38-45. (9) Orkin, B. A. had. Eng. Chem. Prod. Res. Deu. 1969,8,154-160. (10)Schultz, H. F.; Weitkamp, J. H. Ind. Eng. Chem. Prod. Res. Deu. 1972. 21. 46-53. (11)Steijns, M.; Froment, G.; Jacobs, P.; Uytterhoeven, J.: Weitkamp, J. Ind. Eng. Chem. Prod. Res. Deu. 1981,20,654-660. (12)Weitkamp, J. Ind. Eng. Chem. Prod. Res. Deu. 1982,21,55+558.
0887-0624/90/2504-0372$02.50/0
to be superior to conventional amorphous catalysts with respect to acidity, selectivity, and resistance to poisons. High selectivities to isoparaffins were achieved at moderate degrees of conversion. However, the long-chain isoparaffins thus produced were predominantly monobranched; multibranched isoparaffins (preferred highdensity jet fuel componentsls and premium heavy lubricant oilslg)were formed only to a minor extent. A catalyst to achieve both high selectivities and high degrees of branching from long-chain n-paraffins under mild conditions would be highly desirable. Both kinetic and thermodynamic factors influence the isomerization of n-paraffins.20i21Low temperatures favor the formation of isoparaffins; however, in order to carry out the reaction at low temperatures, a strong acidic (essentially superacidic) catalyst is needed. More recently, the increasing demand for high-octane gasoline22 and strong concerns for environmental issues23have driven research toward the direction of isomerized, alkylated,24 and oxygenated25fuels. The high-octane number of iso(13)Weitkamp, J. Appl. Catal. 1983,8,123-141. (14)Martens, J. A.; Jacobs, P. A.; Weitkamp, J. Appl. Catal. 1986,20, 239-281. (15)Martens, J. A.; Jacobs, P. A,; Weitkamp, J. Appl. Catal. 1986,20, 283-303. (16)Giannetto, G. E.;Perot, G.R.; Guisnet, M. R. Ind. Eng. Chem. Prod. Res. Deu. 1986,25,481-490. (17)Giannetto, G.E.;Alvarez, F. B.; Guisnet, M. R. Ind. Eng. Chem. Res. 1988,27,1174-1181. (18)Cookson, D.J.; Lloyd, C. P.; Smith, B. E. Energy Fuels 1987,1, 438-441. (19)Bull,S.; Marmin, A. Proceeding, 10th World Petroleum Congress; Heyden: London, 1980;Vol, 4,p 221. (20)Asinger, F. Paraffins-Chemistry and Technology; Pergamon Press: New York, 1968. (21)Pines, H. The Chemistry of Catalytic Hydrocarbon Conversions; Academic Press: New York, 1981. (22)David, B. C. Oil Gas J. 1989,20,35-40. (23)Unzelman, G. H. Oil Gas J. 1989,15, 33-37. (24)Rvp, Octane Woes Press US. Refiners. Oil Gas J. 1989,15, 22-24. __
(25)Clark, R. G.; Morris, R. B.; Spence, D. C.; Tucci, E. L. Energy Prog. 1987,7,164-169.
0 1990 American Chemical Society
Hydroisomerization and Hydrocracking of n-Paraffins pentane and the extremely valuable isobutane have attracted extensive research on isomerization of n-butane and n-pentane with superacidic (especially solid superacidic) catalysts. Certain hydrogen halides or metal halides supported on graphite,%Dresins,- and metal oxideswa are known to be solid superacids. They can isomerize n-butane and n-pentane at low temperatures, but they are thermally unstable and corrosive. Recently, sulfate ion modified Fe203,=TiO2,= Zr02?7Hf02,%and SnOZNwere found to be solid superacids. Among them, Zr02/S042has been shown to be a very reactive catalyst. The noncorrosive and highly acidic nature of this catalyst has stimulated extensive studies on its use in the isomerization of n-butaneand of n-pentaneU during the past decade. Nevertheless, lack of a hydrogenation function to balance the superacidic function resulted in rapid deactivation of this catalyst. More recently, platinum-impregnated Zr02/S042-was shown to be quite stable under hydrogen pressure for isomerization of n-pentane& and desulfurized light naphtha;46the ratio of isopentane to n-pentane at a given temperature indicated that the reaction was close to equilibrium. We have recently carried out the isomerization of npentane in a pulse fixed bed microreactor with Zr02/S042as catalyst!' Cracking products were also obtained during the reaction, and surprisingly, an extremely high selectivity, e.g., >9890, of isobutane in C4products was observed at temperatures around 90 "C. Evidently, once the isobutane was formed on the superacid site, it desorbed quickly before reaching equilibrium. If the reaction could be applied to heavier n-paraffins, high selectivities to cracked isoparaffins should be obtained. This implies that platinum-doped Zr02/S042-might be a more effective catalyst for hydrocracking of heavier n-paraffinic hydrocarbons than for hydroisomerization of short-chain nparaffinic hydrocarbons in terms of producing short-chain isoparaffins.
(26)Rodewald, P. G. U.S.Patent 3 962 133,1976. (27)Yoneda, N.;Fukuhara, T.; Abe, T.; Suzuki, A. Chem. Lett. 1981, 1485-1488. (28)Magnotta, V. L.;Gates, B. C. J . Catal. 1977,46,266-274. (29)Dooley, K. M.; Gates, B. C. J . Catal. 1985,96,347-356. (30)Krzywicki, A.; Marczewski, M. J . Chem. Soc., Faraday Trans. 1 1980,76,1311-1322. (31)Imamura, H.; Soga, K.; Sato, M.; Wallace, W. E. Chem. Lett. 1980, 957-958. (32)Imamura, H.; Kato, Y.;Tsuchiya, S. Bull. Chem. SOC.Jpn. 1984, 57,2309-2310. (33) . . Imamura., H.:. Wallace. W. E. J. Catal. 1980.64. , . 238-239. (34)Hattori, H.;TakahaskiIO.; Takagi, M.; Tanabe, K. J . Catal. 1981, 68,132-143. (35)Tanabe, K.; Kayo, A.; Yamaguchi, T.,J . Chem. Soc., Chem. Commun. 1981,602-603. (36)Hino, M.; Arata, K. J . Chem. Soc., Chem. Commun. 1979, 1148-1149. (37)Hino, M.; Arata, K. Chem. Lett. 1981,1671-1672. (38)Arata, K.; Hino, M. React. Kinet. Catal. Lett. 1984,25,143-145. (39)Matauhashi, H.; Hino, M.; Arata, K. Chem. Lett. 1988,1027-1028. (40)Hino, M.; Kobayashi, S. J . Am. Chem. SOC. 1979,101,6439-6441. (41)Hino, M.; Arata, K. J . Chem. Soc., Chem. Commun. 1980, 851-852. (42)Stocker, M. J . Mol. Catal. 1985,29,371-377. (43)Yori, J. C.;Luy, J. C.; Parera, J. M. Appl. Catal. 1989, 46, 103-112. (44)Hino, M.; Arata, K. React. Kinet.CataZ. Lett. 1982,19,101-104. (45)Tanabe, K.;Yamaguchi, T. Successful Design of Catalysts;Inui, T., Ed.; Elsevier Science: Amsterdam, 1988. (46)Hosoi, T.; Shimidzu, T.; Itoh, S.; Baba, S.; Takaoka, H.; Imai, T.; Yokoyama, N. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1988, 33, 562-567. (47)Dogan, C.M.S. Thesis, University of Pittsburgh, 1989.
Energy & Fuels, Vol. 4, No. 4, 1990 373 The objective of this research work was to investigate the reaction when long-chain n-paraffiis are hydrocracked on a solid superacid catalyst under mild conditions. The yield of highly branched long-chain paraffins (high-density jet fuel components and premium heavy lubricants) as a function of reaction variables, i.e., temperature, time, and catalyst concentration, was also studied.
Experimental Section Materials. Paraffinic hydrocarbon feedstocks, n-hexadecane (99%) and n-heptane (99%), as well as Hammett indicators (pK, range from -8.2 to -14.52) were obtained from Aldrich Chemicals, Inc. Aqueous ammonium hydroxide (28% NHJ and sulfuric acid (98%) were supplied by Fisher Scientific Co. For catalyst synthesis, ZrOC12.8H20 (99.9985% ), HfCl, (98+ % ), and H,PtC&. 6H60 were obtained from Strem Chemicals, Inc. Catalyst Synthesis. Zirconium (or hafnium) hydroxide was prepared by hydrolyzing an aqueous solution of a zirconium or hafnium salt with aqueous NHIOH a t room temperature. The precipitates were washed with distilled water until no chloride ion was detected in the filtrate and then dried overnight at 110 "C. After drying, the solids were powdered to below 100 mesh. Impregnation of platinum was done by the incipient wetness method. On the basis of the amount of metal hydroxide used, 0.5-0.9 wt % of platinum was introduced to the solid surface, followed by drying a t 110 "C overnight. Sulfation of the hydroxides was carried out by stirring 10 mL of 1 N sulfuric acid per gram of hydroxide in a beaker for 1 h. After filtration, the materials were dried at 110 "C for 2 h and finally calcined at 650 "C for 3 h. The preparation of zirconium-hafnium bimetal oxides was done by dissolving different ratios of both salts in distilled water. The aqueous solution was stirred for 20 min before hydrolyzing with aqueous NHIOH a t room temperature. All other catalyst preparation procedures were carried out as described above. The two types of reactors used in the following experiments are described below. N o n s t i r r e d Batch Minireactor E x p e r i m e n t s w i t h n Heptane. Two stainless steel miniature sample cylinders (1in. 0.d. X 3 21/32 in.) obtained from The Swagelok Companies were used as minireactors to test catalyst activities and to study the effect of moisture with n-heptane as the hydrocarbon feedstock. One gram of each catalyst was activated in an oven at 450 OC for 1.5 h before feeding to the minireactors. After the catalysts were cooled to room temperature, 4 mL of n-heptane were added to and each reactor, which was closed immediately, purged with H2, pressurized to 250 psig of Hz. Both minireactors were then placed in a preheated oven a t the desired temperature for 1 h. After the reaction, both gaseous and liquid products were analyzed by gas chromatography. Horizontal Shaking Microreactor E x p e r i m e n t s w i t h n -Hexadecane. A horizontal shaking microreactor system, providing good catalyst-reactant mixing and gas-liquid mass transfer, was used to perform the hydroisomerization and hydrocracking experiments with n-hexadecane. The microreactor consists of three portions: a horizontal reactor tube (1in. 0.d. x 4.625 in.), a vertical reactor stem (1/2 in. 0.d. x 10 in.), and a multiport valve connected on top of the reactor stem. The horizontal reactor tube is sealed on one end, and the other end is fitted with a cap to allow charging and discharging of the reactant and produds. The reactor stem keeps the multiport valve away from the heat source; it also provides additional hydrogen gas volume for reaction. The multiport valve accommodates a pressure gauge (or transducer), a thermocouple, and a quick connect to allow monitoring of reaction variables as well as charging and discharging of gases. The reactor is mounted on a stand, and a wrist-action shaker provides a horizontal shaking motion. The microreactor was predried in an oven a t 110 "C for 1.5 h, and the desired amount of catalyst was activated in a n oven a t 450 "C for 1.5 h. After the catalyst was placed in the horizontal reactor tube, the open end was immediately closed and the top of the reactor stem was plugged by a fitting. The horizontal portion of the reactor was then immersed into cooling water for 10 min to allow the catalysts to cool to room temperature. After
Wen et al.
374 Energy & Fuels, Vol. 4 , No. 4, 1990 FEED : n-HEPTANE
250 PSlG HYDROGEN
TIME = 1 HR 00 70 -
f
60-
i 0
50-
3-methylpentane
n-hexane
v)
40-
30
-
1.17 0.53
n-heptane
25.46 1.23
c7+
OReaction conditions: 160 "C, 250 psig initial hydrogen, 1 h. Catalyst: Pt/Zr02/S0,2-;feed/catalyst = 2.70.
C
8
Distribution from n -Heptane Conversiono components wt% wt% 4.55 2,2,3-trimethylbutane 1.26 8.90 2,2-dimethylpentane 2.50 0.25 2,3-dimethylpentane 4.26 n-butane isopentane 3.29 2,4-dimethylpentane 4.95 0.37 3,3-dimethylpentane 2.54 n-pentane 2-methylhexane 2,2-dimethylbutane 0.34 18.28 2,3-dimethylbutane 0.53 16.63 3-methylhexane 2-methylpentane 1.92 1.04 3-ethylpentane
Table I. Product components propane isobutane
P REACTION TEMPERATURE, OC
Figure 1. Catalyst reactivity comparison for n-heptane conversion.
cooling, the plug was removed and 8 mL of n-hexadecane was poured in from the top of the reactor stem. As soon as the multiport valve was connected to the reactor stem, the reactor and was mounted to the stand, purged and pressurized with H2, then allowed to shake for 1min before immersion into a preheated fluidized sand bath at the desired temperature. After reaction, the reactor was removed from the stand and cooled to room temperature with water. A Varian 3300 GC equipped with a sampling valve was used to analyze gaseous products. Separation in. 0.d. of gaseous components was done by using a 15 ft x stainless steel column packed with 10% SP-2100 on 801100 supelcoport. The liquid products were analyzed by an HP-5890I1 GC through a 30m X 0.25 mm i.d. DB-1capillary column with an injection split ratio of 50 to 1. The identificationof hydrocarbon components was done by comparing retention times with known standards.
Results and Discussion As mentioned above, nonstirred batch minireactors were used to study catalyst activities as well as the effect of moisture content with n-heptane as feedstock; hydroisomerization and hydrocracking of n-hexadecane were carried out in batch horizontal shaking microreactors. Catalyst Reactivity Comparisons. Three catalysts, P~/ZIO,/SO,~,Pt/Hf02/S0z-, and Pt/ZrO2/HfOz/S0,2(Zr/Hf molar ratio of 1/1)were tested for their activities for n-heptane conversion in the nonstirred batch minireactors. The batch experiments were carried out a t an initial Hzpressure of 250 psig for 1 h with temperatures between 120 and 160 OC. Conversions (wt %) versus reaction temperatures using these three catalysts are shown in Figure 1. At low temperatures, the sulfated bimetal oxide was the most active catalyst for n-heptane conversion, more active than sulfated zirconium oxide. The sulfated hafnium oxide alone showed the lowest activity. However, with increase in reaction temperature, sulfated zirconium oxide increased in activity, exceeding the conversion level of the sulfated bimetal oxide above 130 "C. An example of product distribution at 160 "C with P~/ZIO~/SO,~as catalyst is listed in Table I. At a 74.5 wt % conversion level, a 68.1% selectivity to isoheptanes was observed; among these products a considerable amount (30%) of dibranched and tribranched paraffins was produced. With respect to hy-
Table 11. Comparison between Thermodynamic Equilibrium Values and Experimental Data from n -Heptane Conversion' components n-butane isobutane n-pentane isopentane n-hexane 2-methylpentane 3-methylpentane 2,2-dimethylbutane 2,3-dimethylbutane n-heptane
exDt* 2.7 97.3 10.1 89.9 11.8 42.8 26.0 7.6 11.8
2-methylhexane
23.8 21.6 1.4 3.3 5.5 6.4 3.3 1.6
3-methylhexane
3-ethylpentane
2,2-dimethylpentane 2,3-dimethylpentane 2,4-dimethylpentane 3,3-dimethylpentane 2,2,3-trimethylbutane
33.1
ea uilib' 38.9 61.1 17.7 82.3 9.2 25.9 11.2 42.7 11.0 5.4 16.8
13.1
1.6 14.3 28.3 7.8 9.8 2.9
'The composition for components having the same carbon number is normalized to 100. *Reactionconditions and data from Table I. CCalculated from thermodynamic data given in ref 48. drocracked products, high selectivities to isoparaffins were seen (97% of isobutane, 90% to isopentane, and 88% to isohexanes). Methane and ethane were not found in the gaseous products, which ruled out hydrogenolysis by Pt;12 the formation of methane and ethane was energetically unfavorable under our conditions. In addition, a small amount of higher molecular weight products (>C,) was observed. It is also worthy of note that no olefinic hydrocarbons were found in the products; this is believed due to the high hydrogenation ability of platinum in the presence of H2. Comparisons between thermodynamic equilibrium values and experimental data from n-heptane conversion are shown in Table 11. For hydrocracked products, both isobutane and isopentane exceed equilibrium values. The high selectivity to isobutane (97.3%) is believed due to the fast desorption of this light compound once it is formed on superacid sites and its inability to readsorb in the presence of high concentrations of heavier hydrocarbons, which has been suggested by Flinn et al.5 The selectivity to isohexanes was slightly lower than the equilibrium value, indicating that readsorption of isohexanes could occur on the catalyst surface. In addition, the amount of dibranched isohexanes was lower than expected. For isomerized products, the ratio of 2-methylhexane to 3-methylhexane (1.10) was slightly lower than the equilibrium value (1.28), (48) Rossini, F. D.; Pitzer, K. S.; knett, R. L.; Braun, R. M.; Pimentel, G. C. Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds; Carnegie Press: Pittsburgh, 1953.
Hydroisomerization and Hydrocracking of n-Paraffins
Energy & Fuels, Vol. 4, No. 4, 1990 375 Table 111. Physical Properties of Zr02/SOfo analyses
sulfur, wt
0.88 107.2 0.127 tetragonal
%
BET surface area, m2/g pore volume, mL/g crystalline phase
M
'
\
"The catalyst was calcined in air at 650 "C for 3 h.
Figure 2. Transformation between Lewis acid site and Bramted acid site. and the amount of multibranched isoheptanes was much lower than theoretical, indicating that equilibrium was not reached under the conditions studied. Effect of Moisture Content in the Feed. Anhydrous and moisture-saturated n-heptane samples were used to study the influence of moisture on catalyst reactivity. The anhydrous n-heptane sample was prepared by dehydrating n-heptane in a sealed bottle with 4A molecular sieves that had been activated a t 400 "C for 3 h. The moisture-saturated sample was prepared by exposing as-received nheptane to air for several days, allowing it to become saturated with moisture. The experiments were carried out a t 110 O C in the nonstirred batch minireactors, and the sulfated bimetal oxide, Pt/ZrOz/Hf02/S02-, was chosen as catalyst because of its high reactivity at low temperature. When moisture-saturated n-heptane was used as feed, the conversion (22.1 w t %) was almost twice that (11.8%) of anhydrous n-heptane as feed. Why moisture enhances the hydrocarbon conversion is not well understood, but the model of superacidity generation proposed by Tanabe* and Yori et may provide some explanation. When moisture is absent in the reaction system, metals (Zr or Hf) probably act as Lewis acid sites (see Figure 2). However, steric hindrance caused by the sulfate clusters may make it more difficult for the reactant to access these acid sites. When moisture is present, bonding between oxygen and metal converts Lewis acid sites to Brornsted acid sites, and the oxygen which has a weakly bonded proton then serves as the catalytic center, as shown in Figure 2. Steric hindrance of sulfate clusters on Brornsted acid sites is much less than that of the Lewis acid sites; this may explain why the presence of moisture enhanced the conversion of the heptane. Characterization of the ZrO2/S0>- Catalyst. Since Pt/Zr02/S0f was shown to be the most reactive catalyst for n-heptane conversion, more detailed characterization of this type of catalyst was done. On the assumption that small amounts of Pt on the surface do not have significant influence on the properties of Zr02/S0t-, all of the characterizations were done on the surface free of Pt. The ZrOz/S042-catalyst was characterized by measuring its sulfur content, BET surface area, pore volume, incremental pore area distribution, crystalline phase, and acid strength. Sulfur analysis was done by oxidation of the catalyst and subsequent titration of the gases with an aqueous NaOH solution. Phase determination was done by X-ray diffraction. BET surface area, pore volume, and incremental pore area distribution were done by nitrogen physisorption. Acidity and acid strength were determined by Hammett indicators and subsequent titration with n- butylamine. These properties are listed in Table 111. A relatively low sulfur content, 0.88 w t % , of the catalyst was obtained after calcination at 650 "C for 3 h. The bulk-phase catalyst had a low pore volume, 0.127 mL/g, but had a fairly high surface area, 107 m2/g. The X-ray diffraction pattern of this catalyst showed a tetragonal structure consistent with literature result^.^^^^*^
Table IV. Acidity and Acid Strength of Zr02/SOf
Catalyst"
amt of acid,
amt of acid,
HQ
mmol/g
HO
mmol/g
-8.20 -11.35 -11.99
2.00 1.01 0.56
-12.70 -13.75 -14.52
0.29 0.25 0.18
"The catalyst was calcined in air at 650 O C for 3 h. 8o
t
d 40-
FEEDICAT = 6/1 300 PSlG HYDROGEN TIME = 50 MIN
w
>
3
30-
2ot 'Ot -A-A-
01 I30
I40
150
160
170
1
REACTION TEMPERATURE, OC
Figure 3. Conversion of n-hexadecaneas a function of reaction temperature. The analysis of incremental pore area distribution showed an extremely broad range of mesopores, from 20 to 500 A, for this catalyst; this indicated that diffusion would not be a limitation for n-hexadecane conversion. The acidity and acid strength of ZrOz/S0,2- are shown in Table IV. The catalyst was found to have an Hovalue less than -14.52, showing that it was indeed a solid superacid (100% HzS04,Ho= -12). It was also found that the amount of acidic sites gradually decreased with increase in acid strength. Hydroisomerization and Hydrocracking of n -Hexadecane. Experiments on the conversion of n-hexadecane were carried out in the horizontal shaking microreactor system with Pt/ZrOz/S04z-as catalyst. Effects of reaction temperature, reaction time, and catalyst concentration were studied. Effect of Reaction Temperature. Hydroisomerization and hydrocracking of n-hexadecane as a function of temperature were investigated in the range of 130-170 OC,by keeping a constant reaction time (50 min), catalyst concentration (feed/catalyst = 6/1), and 300 psig of initial H2 (49) Yamaguchi, T.; Tanabe, K. Mater. Chem. Phys. 1986,16,67-77. (50) Sohn, J. R.; Kim, H. W. J. Mol. Catal. 1989,52, 361-374.
Wen et al.
376 Energy & Fuels, Vol. 4,No. 4,1990
CAT : P ~ I Z ~ O ~ I S O ~ ~ .
FEEDICAT 611 300 PSlG HYDROGEN 40.' TIME = 50 MIN
'O/
'O/
E
c5-cY0
30.
9w>
'O/
CAT : P ~ / z ~ o ~ / s o ~ ~ -
FEEDKAT 611 300 PSlG HYDROGEN TIME = 50 MIN
01 130
140
150
160
171)
I30
REACTION TEMPERATURE, OC
140
150
160
I70
REACTION TEMPERATURE, OC
Figure 4. Product yields from conversion of n-hexadecane.
Figure 6. Multibranched to monobranched ratios of long-chain isoparaffins from conversion of n-hexadecane. CAT : P ~ / z ~ o ~ I s o ~ * FEEDiCAT = 611 300 PSlG HYDROGEN TIME = 50 MIN
/o
FEEDICAT = 611 300 PSlG HYDROGEN TIME = 50 MIN
75
130
140
150
160
I70
REACTION TEMPERATURE, OC
Figure 5. Selectivities of isoparaffins from hydrocracking of
n-hexadecane.
pressure. The results are shown in Figures 3-8. With Pt/Zr02/S042-as catalyst, total conversion increased from 55.7 to 72.7 wt % when the temperature was increased from 130 to 170 "C BS seen in Figure 3. Sulfated zirconium oxide catalyst with no added Pt was also studied. As shown in Figure 3, when Pt was absent on the catalyst surface, the catalyst deactivated rapidly, losing its reactivity almost completely. Fast deactivation of the same type of catalyst on n-pentane conversion in a fixed bed reactor system was also observed by Hoisi et al.46 It is known that the balance between the hydrogenation function and the acid function is important for acid catalysts. The addition of Pt further indicated the necessity of balancing both functions when using solid superacids as catalysts.
130
140
150
160
170
REACTION TEMPERATURE, OC
Figure 7. Cracking and isomerization selectivities of n-hexadecane. Although n-hexadecane conversion did not change greatly in the temperature range studied, the product distribution did change significantly (Figure 4); a rapid increase in the C5-C9 gasoline range product yield from 21 w t % at 130 "C to 40.8 w t % at 170 "C was seen. The C, and Clo-C13 fractions increased slightly, while isohexadecanes decreased considerably. No products above c 1 6 were detected. Selectivities to isoparaffins for C,, C5-Cg, and C1o-C13 fractions are shown in Figure 5. Very high selectivities, i.e., >go%, to isoparaffins were observed for all fractions. The high selectivity of isobutane in the C4 fraction indicates that the cracking reaction is predominantly governed by kinetics. n-Hexadecane must isomerize before the molecule is cracked. However, gradual
Hydroisomeritation and Hydrocracking of n-Paraffins
Energy & Fuels, Vol. 4, No. 4, 1990 377
CAT : P ~ / z ~ o ~ I s o ~ * FEEDEAT = 611 300 PSI0 HYDROGEN TIME = 50 MIN
n
o IW
n W
0 0
0
a vi
t;
0 3
A
90-
! !l W
B
A
A-
c4
u)
0
g
zU q a
Y
85-
it
V
8
CAT : P V Z ~ O ~ / S O ~ ~ FEEDICAT = 611 300 PSI0 HYDROGEN TEMP = 150°C
80 -
CARBON NUMBER
Figure 8. Distribution of cracking products from n-hexadecane. 80 FEEDEAT
75 I
611
15
0
30
50
m
REACTION TIME, MIN
TEMP = 15OoC
Figure 10. Selectivities of isoparaffins from hydrocracking of
n-hexadecane.
60
60 O'CONV
/ /
i
i
p
*5*2 2.0 -
-40
W
> 2
-30
8
1.5
-
1.00
REACTION TIME, MIN
Figure 9. Conversion and product yields of n-hexadecane as a function of reaction time.
decrease in isoparaffin selectivities with increasing temperature indicates that thermodynamics still play a minor role for the cracking reactions in the temperature range studied. The degree of branching for long-chain paraffinic products is shown in Figure 6. For cracking products in the ClO-Cl3 range, more than 50% were multibranched (dibranched and further branched) isoparaffins. The degree of branching was even more pronounced for isohexadecanes; up to 70% of isohexadecane components were multibranched isoparaffins, products suitable either as high-density jet fuel components or as base materials for lubricants. Cracking and isomerization selectivities as a function of reaction temperature are shown in Figure 7. A moderate increase in cracking products and decrease in isohexadecanes occur with increased temperature. A comparison of product distribution between low (130 "C) and high (170 "C) temperatures, due to cracking, is shown in Figure 8. Methane and ethane were not found in the gaseous products; also no CI4or C15paraffins were found in the liquid products. Only a small amount of propane and a corresponding amount of tridecanes were
0
I' -L
CAT : P ~ I Z ~ O ~ I S O ~ ~ .
0.5.
v
FEEDKAT = 611 300 PSlG HYDROGEN TEMP = 150°C
15
x)
50
75
REACTION TIME, MIN
Figure 11. Multibranched to monobranched ratios of long-chain isoparaffins from convemion of n-hexadecane.
produced. At 130 "C, the cracking products were more evenly distributed between C4and Clz. However, when the reaction temperature was increased to 170 "C, a significant increase in butanes (mostly isobutane) and gasoline range components was seen. Effect of Reaction Time. n-Hexadecane conversion as a function of reaction time was studied in the range of 15-75 min at constant temperature (150 "C), catalyst concentration (feed/catalyst = 6/ l),and 300 psig initial hydrogen pressure. The results are shown in Figures 9-12. A sharp increase in the total conversion of n-hexadecane occurred within 30 min of reaction time (Figure 9). Conversion continued to increase for 50 min followed by a slower increase thereafter. n-Hexadecane conversion
Wen et al.
378 Energy & Fuels, Vol. 4, No. 4, 1990
I
300 PSlG HYDROGEN
80-
-80
70-
- 70
60-
'8
d
300 PSlG HYDROGEN TEMP 15OoC 50- TIME = 50 MIN
0.
I= 50
1I
I
1 15
30
50
75
REACTION TIME, MIN
FEEDEATALYST
Figure 12. Cracking and isomerization selectivities of n-hexadecane.
Figure 13. Conversion and product yields of n-hexadecane as a function of catalyst concentration.
reached 73.0 w t % after a reaction time of 75 min, which is about the same conversion level compared with that at 170 "C and a reaction time of 50 min; a higher temperature is not necessary to obtain high conversions using the superacid catalyst. Although the conversion did not increase sharply after 30 min, a rapid increase in the C6-C9 gasoline range products occurred between 30 and 75 min reaction time (Figure 9). The yield of isohexadecanes increased rapidly within 30 min, but then began to decrease. Both yields of C4 and Cl0-Cl3 fractions increased moderately within the time range studied. The above observations lead us to believe that formation of C5-C9 products was mainly due to cracking of the initial isomerized isohexadecanes. Figure 10 shows a very high selectivity, over 90% of isoparaffins, for all three fractions of hydrocracked products; this selectivity decreased slightly with increase in reaction time. The high isoparaffin selectivity, even at short reaction times, e.g., 15 min, supports the premise that the cracking reaction is predominantly controlled by kinetics. Figure ll shows that the multibranched to monobranched ratio of isohexadecanesincreases rapidly with increasing reaction time, but only a slight increase in that of the C1o-C13 fraction was seen. This observation could be interesting from a process point of view; although the yield of isohexadecanes does decrease to a minor extent (Figure 9), the significant increase in the degree of branching makes the products more valuable. Cracking and isomerization selectivities are shown in Figure 12. The increase in cracking products and decrease in isohexadecanes with increase in reaction time again shows the same pattern as that of the reaction temperature effect. Effect of Catalyst Concentration, The hydroisomerization and hydrocracking of n-hexadecane as a function of catalyst concentration were investigated with feed/catalyst ratios between 2 / 1 and 12/1, at constant reaction temperature (150 "C), reaction time (50 min), and 300 psig initial H2pressure (Figure 13). As seen from Figure 13, total conversion increased significantly, from 57.0 to 75.9 w t %, with increase in catalyst concentration within the range studied. This observation indicates that, owing to the small number of acid sites
available on the surface of Pt/ZrO2/S02-, conversion was limited by mass transfer between the reactant molecules and the acid sites. Between a feed/catalyst ratio of 12/1 and 6/1, the yields of isohexadecanes, C1o-C13 hydrocarbons, and C4fractions did not change significantly, but a rapid increase in the yield of the C,-C9 fraction was observed. When the catalyst concentration was further increased, sharp increases in the yield of the C5-C9fraction, coupled with a decrease in that of isohexadecanes, were seen, with only a slight increase in the yield of butanes (mostly isobutane). It is widely accepted that superacid catalyzed hydrocarbon conversion proceeds through highly reactive and positively charged intermediates, e.g., carbonium ions. The very high yields of hydrocracked isoparaffins leads us to believe that hydrocracking of n-hexadecane is preceded by hydroisomerization. Monobranched isohexadecanes are formed through skeletal isomerization of n-hexadecane, and multibranched isohexadecanes are formed through consecutive reactions. @-Scissionof a tertiary carbonium ion leading to a small tertiary carbenium ion is more energetically favored than formation of secondary or primary carbenium ions. The absence of methane and ethane in the cracking products excludes the possible formation of primary carbenium ions by @-scission,which excludes the reaction pathway involving hydrocracking of monobranched isohexadecanes under the conditions studied. Secondary or tertiary carbenium ions can be produced by @-scissionof multibranched paraffinic intermediates, and these secondary carbenium ion intermediates can undergo either skeletal isomerization (favored) to form more stable tertiary carbenium ion intermediates or hydride abstraction (less favored) to form smaller n-paraffins. @-Scission of multibranched isoparaffins has been postulated by Froment" and Martens et al.14 and is consistent with the product distribution obtained by hydrocracking of nhexadecane in the present work. In the overall carbonium ion mechanism, both Pt (on the superacid surface) and hydrogen gas are important to complete the network cycle. Pt hydrogenates olefinic components (from @-scissionof carbenium ions) to prevent
Energy & Fuels 1990,4, 379-384
catalyst deactivation, while hydrogen gas acts either as a hydride donor to saturate carbenium ion intermediates or as a proton donor to reactivate Brernsted superacid sites. Acknowledgment. We acknowledge Celal Dogan and Joseph L. Fabec for their assistance in the characterization of catalysts. We also thank Dan Fraenkel for discussions. The research was supported by the U.S. Department of Energy under University Research Program Contract No. DE-FG22-87PC79928,
379
Registry NO.2102,1314-23-4; HfO2,37230-85-6;Pt,7440-06-4; hafnium zirconium oxide, 104365-48-2; hexadecane, 544-76-3; heptane, 142-82-5;propane, 74-98-6; ieobutane, 75-28-5; butane, 106-97-8; isopentane, 78-78-4; pentane, 109-66-0; 2,a-dimethylbutane, 75-83-2; 2,3-dimethylbutane, 79-29-8; 2-methylpentane, 107-83-5;3-methylpentane, 96-14-0; hexane, 110-54-3; 2,2,3-trimethylbutane, 464-06-2; 2,2-dimethylpentane, 590-35-2; 2,3-dimethylpentane, 565-59-3; 2,4-dimethylpentane, 108-08-7; 3,3dimethylpentane, 562-49-2; 2-methylhexane, 591-76-4; 3methylhexane, 589-34-4; 3-ethylpentane, 617-78-7.
Coal Solubility and Swelling. 1. Solubility Parameters for Coal and the Flory x Parameter Paul C. Painter,* John Graf, and Michael M. Coleman Polymer Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received January 30, 1990. Revised Manuscript Received May 29, 1990
This is the first of three papers that deal with coal solubility and swelling. An association model has been applied to coal, and this predicts that the mixing of coal with certain solvents will be determined by a balance between unfavorable "physical" interactions, measured by a Flory x parameter, and favorable hydrogen bonding interactions. This first paper deals with the determination of x from solubility parameters. Methods based on group contributions and swelling measurements are assessed. The former suffers from large errors while the latter does not give correct values because of large free-volume differences between coal and most solvents. Even though group contribution methods are flawed when applied to coal, they at least appear to give values in the right range.
Introduction The "solubility" &e., the amount of soluble material that can be extracted from a particular solvent at a given temperature) and swelling characteristics of a coal are fundamentally determined by its molecular structure. In order to relate macroscopic properties to molecular characteristics, however, we require a sound theoretical model. A considerable degree of at least qualitative insight'* has been obtained by applying the theories of Paul Flory,'oJ1 particularly those describing polymer solution thermodynamics and the swelling of cross-linked rubbers, and it is now generally accepted that coals can be described as macromolecular networks. Nevertheless, it is also widely recognized that these theories have serious shortcomings, when applied to coal. Perhaps the most crucial of the additional difficulties are that, first, the theories only deal with weak interactions (dispersive forces), which are handled by the assumption of random contacts and the definition of a parameter x. Strong specific interactions, such as the hydrogen bonds that occur in most coals, cannot be accounted for in this manner. Second, the "chains" that are presumed present in coal are probably too short and too stiff to obey Gaussian statistics, so that even if Flory's theories of swelling are correct, and this is an area of some controversy, they would be inapplicable to coal. Flory's work has been modified by various groups to account for these problems. For example, Kovac12has developed a modified Gaussian model that has been ap-
* To whom correspondence should be addressed. 0887-0624/90/2504-0379~02.50 ,I O I
,
plied to the swelling of coal by Larsen et ala5Recent work in this laboratory has been concerned with the use of association models to account for hydrogen b~nding.'~-'~ Our purpose in this series of papers is to put this work together in a comprehensive fashion and examine its strengths and limitations. This will lead us to the prediction of so far unobserved behavior, which ultimately allows a test of the validity of our approach, and to a reassessment of the way we interpret swelling measure(1) van Krevelen, D. W. Coal; Elsevier: New York, 1981. (2) van Krevelen, D. W. Fuel 1966, 45, 229. (3) Green, T.; Kovac, J.; Brenner, D.; Larsen, J. W. In Coal Structure; Meyers, R. A., Ed.; Academic: New York, 1982. (4) Larsen, J. W. In Chemistry and Physics of Coal Utilization; Cooper, B. R., Petrakis, L., Eds.; AIP Conference Proceedings 70; AIP New York, 1981. (5) Larsen, J. W.; Green, T. K.; Kovac, J. J.Org. Chem. 1986,50,4729. (6) Lucht, L. M.; Peppas, N. A. In Chemistry and Physics of Coal
Utilization; Cooper, B. R., Petrakis, L., Eds.;AIP Conference Proceedings 18; AIP: New York, 1981. (7) Lucht, L. M.; Peppas, N. A. Fuel 1987,615, 803. (8) Lucht, L. M.; Peppas, N. A. J. Appl. Polym. Sci. 1987,33,2777. (9) Brenner, D. Fuel 1985, 64, 167. (IO) Flory, P. J. Principles of Polymer Chemistry; Comell University
Press: Ithaca: NY, 1953. (11) Flory, P. J. Selected Works of Paul J. Flory; Mandelkern, L., Mark, J. E., Suter, U. W., Yoon, Do. Y., Eds.;Stanford University Prese: Stanford, CA, 1985; Vol. 1-111. (12) Kovac, J. Macromolecules 1978, 11, 362. (13) Painter, P. C.; Park, Y.; Coleman, M. M. Macromolecules 1988, 21. 66. (14) Painter, P. C.; Park, Y.; Coleman, M. M. Macromolecules 1989, 22, 570. (15) Painter, P. C.; Park, Y.; Coleman, M. M. Macromolecules 1989, 22, 580. (16) Painter, P. C.; Park, Y.; Coleman, M. M. Energy Fuels 1988,2, 693.
0 1990 American Chemical Society
