Energy & Fuels 1993, 7, 960-967
960
High-pressure Thermal Cracking of n-Hexadecane in Tetralin Farhad Khorasheh and Murray R. Gray' Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T 6 G 2G6 Received April 26, 1993. Revised Manuscript Received August 12, 199P
Thermal cracking of n-C16 in tetralin was carried out in a tubular flow reactor at 400-460 "C and 13.9 MPa. Initial mole fractions of n-Cl6 in the feed were 0.01,0.03, and 0.05. Major n-(&-derived products were C1 to C14 n-alkanes and C2 to C15 a-olefins. Other major products were 1- and 2-alkyltetralins. Product distributions were nearly equimolar and total n-c16-derived product selectivities were close to 200 mol per 100 mol of n-Cu decomposed. Product distributions were dependent on n-Cl6 conversion. Selectivities for a-olefins decreased with increasing n-Cl6 conversion. Total n-alkane selectivities were in excess of 100 mol per 100 mol of n-C16 decomposed and increased with increasing n-C16 conversion. Apparent first-order rate constants for overall conversion of n-C16 in tetralin decreased with increasing n-C16 conversion and were lower than corresponding values for thermal cracking of pure n-C16. Alkyl radicals generated from the decomposition of hexadecyl radicals preferentially abstracted hydrogens from tetralin rather than n-C16, which inhibited the chain mechanism for conversion of n-C16 and resulted in equimolar distribution of n-alkanes. Hydrogen abstraction from tetralin involved hydrogens in both a- and @-positionsof the saturated ring, leading to the formation of 1- and 2-tetralyl radicals. These radicals participated in addition reactions with a-olefins leading to the formation of 1-and 2-alkyltetralins as major products. a-Olefins were also reduced to give additional n-alkanes. Possible reactions leading to the formation of alkyl radicals from a-olefins include addition reactions involving hydrogen atoms generated from decomposition of tetralyl radicals and hydrogen transfer from tetralin and tetralin-derived free radicals to a-olefins. Depending on the reaction conditions, however, addition of tetralyl radicals to a-olefins was 1.5-15 times faster than reduction of a-olefins to corresponding n-alkanes based on the molar ratio of alkyltetralins to additional n-alkanes in the products.
Introduction Tetralin has long been recognized as an effective hydrogen donor solvent. Numerous studies have appeared in the literature where feedstocks or representative model compounds were processed in tetralin as a donor solvent with or without added hydrogen or a catalyst. Some of these donor solvent studies include thermal hydrogenation' and visbreaking2 of crude residua and hydrocracking of alkylaromatics.3 Tetralin has also been used as a donor solvent in coal liquefaction studies (for example, Curran et aL4 and Vernon5) and in model compound studies because it represents hydroaromatics in hydrogenated coal distillates. Tetralin is useful as a donor solvent because its hydrogens, in particular those in the a-position of the saturated ring, can be readily abstracted by radicals generated during thermal processing of feedstocks. Fur-
ther dehydrogenation of tetralyl radicals to 1,2-dihydronaphthalene and eventually to naphthalene results in a net hydrogen transfer from tetralin to the feedstock. High-pressure thermal cracking of n-Cu by itselP and in the presence of alkylaromtic solvent' were reported elsewhere. High-temperature (600 "C+), low-pressure (atmospheric) pyrolysis of n-alkanes has been studied extensively in the pyrolysis literature since the early work by Rice, Herzfeld, and Kossiakoff.alo In the pyrolysis of n-Cl6 under these conditions, for example, alkyl radicals undergo successive(multistep) decompositionby @-scission (Rice-Kossiakoff mechanism) leading to the formation of methane and ethane as the only saturated products and C2 to C15 a-olefins. The product distribution for lowpressure thermal cracking of n-Cl6 has been verified by several investigator~.ll-~3 A t high pressures, bimolecular reactions (radical addition and hydrogen abstraction) are ~~~
* Author for correspondence.
Abstract published in Advance ACS Abstracts, October 1, 1993. (1)Carbon, C. S.;Langer, A. W.; Stewart,J.; Hill, R. M. Thermal Hydrogenation: Transfer of Hydrogen from Tetralin to Cracked Residua. Znd. Eng. Chem. 1968,50, 1067-1070. (2)Langer, A. W.; Stewart,J.; Thompson, C. E.; White, H. T.; Hill, R. M. Hydrogen Donor Diluent Visbreaking of Residua. Znd. Eng. Chem. Process Des. Dev. 1962,1, 309-312. (3) Hill, J. C.; Engelbrecht, R. M.; Moore, R. N.; Spillane, L. J. Hydrocracking of Alkylaromatic Compounds with Hydrogen Donors. Prepn.-Am. Chem. Soc., Diu. Pet. Chem. 1968, 13(4), 183-192. (4) Curran, G. P.; Struck, R. T.; Gorin, E. Mechanism of HydrogenTransfer Process to Coal and Coal Extract. Znd. Eng. Chem. Process Des. Dev. 1967, 6, 166-173. (5)Vernon, L. W. Free-Radical Chemistry of Coal Liquefaction: Role of Molecular Hydrogen. Fuel 1980,59, 102-106.
~~
(6) Khorasheh, F.; Gray, M. R. High-pressure Thermal Cracking of
n-Hexadecane Znd. Eng. Chem. Res., in presa. (7)Khorasheh, F.; Gray, M. R. High Pressure Thermal Cracking of n-Hexadecane in Arometic Solvents. Znd. Eng. Chem. Res., in press. ( 8 ) Rice, F. 0. The Thermal Decomposition of Organic Compounds from the Standpoint of Free Radicals. III. The Calculation of the Produds from ParaffinHydrocarbona. J. Am. Chem. SOC. 1933,55,30353040. (9)Rice, F.0.; Herzfeld, K. F. The Thermal Decomposition of Organic Compounds from the Standpoint of Free Radicals. IV. The Mechanism 1934,56, 284-289. of some Chain Reactions. J. Am. Chem. SOC. (10)Kossiakoff, A.; Rice, F. 0. Thermal Decomposition of Hydrocarbons, Resonance Stabilization and Isomerization of Free Radicals. J . Am. Chem. SOC.1943,65, 59+595. (11)Voge, H. H.; Good, G. M. Thermal Cracking of Higher Paraffins. J. Am. Chem. SOC.1949, 71, 593-597.
0887-0624/93/2507-0960$04.00/00 1993 American Chemical Society
Thermal Cracking of n-Hexadecane in Tetralin favored over the unimolecular radical decomposition. Radical decomposition reactions also have higher activation energies than bimolecular reactions and are favored at higher temperatures. At low temperatures and high pressures, higher alkyl radicals also participate in bimolecular hydrogen abstraction reactions to give higher alkanes as major reaction products.13 Fabuss, Satterfield, and Smith13J4 proposed a single-step mechanism for alkanes pyrolysisat high pressures (1-7 MPa) where parent radicals undergo a single-step decomposition, and the resulting lower alkyl radicals participate in hydrogen abstraction to give the corresponding n-alkanes. The single-step decomposition of n-alkanes is favored a t high pressures and relatively mild temperatures (350-450 "C) and is characterized by nearly equimolar distribution of n-alkanes and a-olefins in the products. Mushrush and Hazlettl5 suggested that for an intermediate pressure of 0.7 MPa, a two-step decomposition adequately represented the observed product distribution from n-Cu cracking at 450 "C. Under high pressures and relatively mild temperatures, radical addition reactions become significant. In high-pressure (13.9 MPaP and liquid-phase16thermal cracking of n-C16 at 380-450 "C, for example, addition of parent hexadecyl radicals to a-olefins led to the formation of alkylhexadecanes as major products. In high-pressure thermal cracking of n-Cl6 in alkylaromatic solvents, addition of solvent radicals to a-olefins gave alkylbenzenes as major products. The main objective of the present study was to examine thermal cracking of n-Cl6 in the presence of a hydrogen donor solvent, namely tetralin, with emphasis on the fate of a-olefins which are produced as primary reaction products. Previous Studies on Thermal Crackingof Tetralin. The reaction mechanisms for thermal cracking of tetralin by itself has been investigated in some donor solvent studies to establish the "base-line" reactions of the solvent. Thermal cracking of tetralin has been investigated under high-temperature (700-900 "C), gas-phase (atmospheric pressure) conditions as well as low-temperature (400-500 "C), high-pressure (10 MPa and higher) conditions. In a recent review, Poutsma17gave the following major classes of products from thermal cracking of tetralin: (1) ring isomerization to methylindan, indan, and indene; (2) dehydrogenation to 1,2-dihydronaphthalene and naphthalene; (3) ring-opening to butylbenzene and other alkylbenzenes; (4) cracking to benzocyclobutene, styrene, and ethylbenzene. The formation of these products can be described in terms of free-radical reactions illustrated in Figure 1. In thermal cracking of tetralin, free radicals can be generated by initiation steps involving the cleavage of the 0 C-C bond (reaction 1)to give a diradical, or the cleavage of the a C-H bond to give 1-tetralyl radical and a hydrogen atom (reaction 2). In high-temperature (12)Depeyre, D.; Flicoteam, C.; Chardaire, C. Pure n-Hexadecane Thermal Steam Cracking. Ind. Eng. Chem. Process Des. Dev. 1985,24, 1251-1258. (13)Fabuss, B. M.; Smith, J. 0.; Satterfield, C. N. Thermal Cracking of Pure Saturated Hydrocarbons. Ado. Pet. Chem. Ref. 1964,9,157-201. (14)Fabuss, B. M.; Smith, J. 0.;Lait, R. I.; Borsanyi, A. S.; Satterfield, C. N. Rapid Thermal Cracking of n-Hexadecane at Elevated Pressures. Ind. Eng. Chem. Proceas Des. Dev. 1962,1,293-299. (15)Mushrush, G. W.; Hazlett, R. N. Pyrolysisof Organic Compounds Containing Long Unbranched Alkyl Groups. Ind. Eng. Chem. Fundam. 1984,23,288-294. (16)Ford, T. J. Liquid Phase Thermal Decompositionof Hexadecane: Reaction Mechanisms. Ind. Eng. Chem. Fundam. 1986,25,240-243. (17)Poutama, M. L.Free-Radical Thermolysis and Hydrogenolysisof Model Hydrocarbons Relevant to Processingof Coal. Energy Fuels 1990, 4,113-131.
Energy &Fuels, Vol. 7, No. 6,1993 961
(4)
@ -@+..
(9)
Figure 1. Some free-radical reactions in thermal cracking of tetralin.
dissociation of tetralin,la radicals generated from these initiation reactions resulted in the formation of benzocyclobutene, o-allyltoluene, and 1,2-dihydronaphthalene as primary products via reactions 3,4, and 5, respectively. The latter could also be formed by radical disproportionation involving tetralyl radicals (reaction 6). Tetralyl and hydronaphthyl radicals could be generated by reverse radical disproportionation (RRD) involving 1,Zdihyd r ~ n a p h t h a l e n e which ~ Q ~ ~is~usually present in tetralin as an impurity (reactions 7 and 8). 1-and 2-Hydronaphthyl radicals could also be generated by hydrogen abstraction from 1,2-dihydronaphthalene. These radicals could decompose to give naphthalene (reaction 9). l-Methylindane radicals can be formed from 2-tetralyl radicals by 1,2-aryl shift.21 Ring opening of tetralin to a butylbenzene radical proceeds via hydrogen atom addition to form a cyclohexadienyl intermediate (reaction 10) and is enhanced in the presence of added hydrogen.22 Butylbenzene radicals can decompose to give benzene and other alkylbenzene~.~~ Product distributions from decomposition of tetralin depend on the reaction conditions. Under high-temper(18)Comita,P. B.;Berman,M.R.;Moore,C.B.;Bergman,R. G.LaserPowered Homogeneous Dissociation of Tetralin. J.Phys. Chem. 1981, 85,3266-3276. (19)Allen, D. T.;Gavalas, G. R. Kinetics of Dialin Thermolysis. Int. J. Chem. Kinet. 1983,15, 219-233. (20)Franz, J. A.; Camaioni, D. M.; Beishline, R. R.; Dalling, D. K. Producta, Radical Intermediates, and Hydrogen Production in the ThermalDecomposition of 1,2-Dihydronaphthalene. J.Org. Chem. 1984, 49,3563-3570. (21)Franz, J.A.;Camaioni,D. M.RadicalPathwaysofCoalDisso1ution in Hydrogen Donor Media. 2. Scissionand 1,2Aryl Migration Reactions of Radicals Derived from Methyindans and Tetralin at 327-627 OC. J. Org. Chem. 1980,45,5247-5255. (22)Penninger,J. M. L. New Aspectaofthe Mechanismfor theThermal Hydrocracking,of Indan and Tetralin. Int. J. Chem. Kinet. 1982, 14, 761-780. (23)Hooper,R. J.;Battaerd, H.A. J.;Evans,D. G.ThermalDissociation of Tetralin Between 300 and 450 OC. Fuel 1979,58,132-138.
Khorasheh and Gray
962 Energy & Fuels, Vol. 7, No. 6, 1993 to Vacuum M P
+"%/HZ
Cylinders
hA
h A
w
Liquid Sampling
W
Feed FUnp
Oven
Figure 2. Schematic diagram of the equipment.
ature, gas-phase condition^,^^ dehydrogenation was the main decomposition path giving naphthalene and 1,2dihydronaphthalene as major products. Under lowtemperature, dense-phase ~ o n d i t i o n sisomerization , ~ ~ ~ ~ ~ to 1-methylindan,and ring opening to various alkylbenzenes, became significant. Methods and Materials Tetralin (99%) and n-hexadecane (99.9%) were obtained from Aldrich. Thermal cracking reactions were carried out in a tubular flow reactor. The apparatus was designed after Broderick.26 A schematic diagram of the equipment is given in Figure 2. The apparatus was designed to feed a liquid, saturated with a dissolved gas, to the reactor for thermal hydrocracking experiments. Saturation was achieved by means of a magnetic circulating pump27 by recycling the gas from the top of the saturator through the liquid. Once saturation was achieved, the circulating pump was isolated from the saturator and the liquid containing the dissolved gas was pumped to the reactor. In the case of high-pressure thermal cracking of n-Cl6 in tetralin, the solution was fed to the 1000-mL saturator and subsequently pressurized with NZ to a pressure about 1 MPa lower than the desired reaction pressure. The reactants were fed to the reactor by means of a high-pressure pump. The reactor was made up of two 30-cm segments of 1/4 in. o.d., 4 mm i.d., glass-lined stainless steel tubing which were connected in series with 1/16 in. o.d., 0.7 mm id., glass-lined stainless steel tubing. The feed line to the first segment and the product line from the second segment were 10-cm segments of the 1/16 in. glass-lined tubing. Stainless steel Swagelokfittings were used for connections. Total reactor volume, including the fittings, was 8.0 mL. (24) Bredael, T.; Vinh, T. H. Pyrolysis of Hydronaphthalenes. 1. Pyrolysis of Tetralin, 1,2-Dihydronaphthalene,and 2-Methylindene. Fuel 1979,58, 211-214. (25) de Vlieger, J. J.; Kieboom, A. P. G.; van Bekkum, H. Behaviour of Tetralin in Coal Liquefaction: Examination in Long-Run BatchAutoclave Experiments. Fuel 1984,63,334-340. (26) Broderick,D. H. High-pressure Reaction Chemistry and Kinetics Studies of Hydrodesulfurization of Dibenzothiophene catalyzed by Sulfided CoO-MoO$y-A1203. Ph.D. Thesis, University of Delaware, 1980. (27) Ruska, W. E. A.; Hurt, L.; Kobayashi, R. Circulating Pump for high pressureand-200to +400 "CApplication. Rev. Sci. Instrum. 1970, 41(10), 1444-1446.
The fittings were the only metal surface that the reactants were exposed to under reaction conditions. These provisions minimized the contact between the reacting media and stainless steel which may result in some catalytic activity. The reactor was placed inside an air bath oven. The temperature of the oven was monitored and controlled to within f l "C of the desired temperature. The length to diameter ratio for the reactor was approximately 150 and the surface to volume ratio was 1000 m2/m3which would allow for a high surface area for heat transfer to maintain near-isothermal conditions with the air bath. Although a thermocouple was not placed inside the reactor, heattransfer calculations28 indicated that, for most experiments, the reaction temperature would rapidly increase from room temperature to within 1 "C of the oven temperature before the end of the 10-cm segment of the 1/16 in. line at the reactor inlet. The volume of this inlet line was only 0.5 % of total reactor volume. Furthermore, thermal cracking reactions are weakly endothermic (AHR" = 19.5 kcal/mol), hence eliminating the occurrence of hot spots inside the tubular reactor. Heat-transfer calculations indicated that the temperature difference between the reactor contents and the air bath due to the endothermic cracking of n-Cl6 was between 0.05 and 0.3 "C, which is within experimental error of the thermocouple measurements. The products were collected in either of two 500-mL receivers. Transient products were accumulated in receiver no. 1. Product flow was diverted to receiver no. 2 when steady state was achieved. The pressure inside the receivers and the reactor was maintained a t a desired level by means of a back pressure regulator. At the end of the experiment, a sample was withdrawn into an evacuated sampling bomb. The product gas was analyzed for C1 to Csgases by gas chromatography (GC). Hydrogen content of the product gas was not measured. The procedure and conditions for gas analysis are outlined elsewhere.29 Liquid products were analyzed by another GC equipped with a DB-1 capillary column and a flame-ionization detector. The following temperature program was used for this GC; initial oven temperature of 35 "C for 3 min, heating up to 300 "C at 5 "C/min, and a final temperature of 300 "C until elution was completed. A small amount of biphenyl (typically 0 . 1 4 2 % of n-C16 by weight) that was added to the feed served as internal standard. Results and Discussion Thermal cracking of n-Cl6 in tetralin was carried out in the tubular flow reactor at 400-460 "C and 13.9 MPa. Initial mol fractions of n-C16 in the feed were 0.01,0.03, and 0.05. For most experiments, feed flow rates were selected to give n-Cl6 conversions below 105%. Fractional conversions of tetralin were lower than fractional conversions of n-C16 by approximately a factor of 2. The relatively large number of moles of tetralin reacted per mole of n-Cl6 decomposed suggest that, under the conditions employed in this study, tetralin does not merely serve to cap free radicals. Three major products namely 1-methylindan, naphthalene, and n-butylbenzene represented approximately 90% of total tetralin-derived prod(28) Khorasheh, F. High Pressure Thermal Cracking of n-Hexadecane. Ph.D. Thesis, University of Alberta, 1992. (29) Chung, S. Y. K. Thermal Hydroprocessing of Heavy Gas Oils. MSc. Thesis, University of Alberta, 1982.
Thermal Cracking of n-Hexadecane in Tetralin ucts (excluding alkyltetralins which were formed by addition of tetralyl radicals to a-olefins). Thermal Cracking of Tetralin by Itself. A number of thermal cracking experiments were performed using tetralin by itself at typical reaction conditions (13.9 MPa, 430-450 "C, 7 = 1h) employed for thermal cracking of n-Cla in tetralin. Chromatographic analysis of the feed tetralin indicated the presence of naphthalene, cis- and trans-decalin, and an unidentified compound as major impurities. These impurities accounted for approximately 1%of the feed by weight. The unidentified compound remained unchanged in the liquid products while additional naphthalene was produced and cis-decalin isomerized to the more stable trans isomer. The major product from thermal cracking of tetralin was l-methylindan 128.71 where the number inside brackets next to product i represent the molar ratio of product i to n-butylbenzene in the reaction products. Other products, in decreasing selectivities, were naphthalene (adjusted to account for feed impurity) [3.341, n-butylbenzene [1.001, an unidentified product (possibly indan or indene) 10.631, ethylbenzene [0.48], propylbenzene 10.193, toluene 10.131,benzene 10.051, styrene [0.04], and allylbenzene [0.031. Minor products included trace quantities of some unidentified Clo compounds, possibly disubstituted benzenes resulting from decomposition of tetralyl radicals (for example, in reaction 4) and a mixture of C20 compounds f0.851 possibly resulting from assorted radical combinations or addition of tetralyl radicals to 1,2-dihydronaphthalene. Franz et aL20identified the CZO compounds produced from thermal decomposition of 1,2dihydronaphthalene. The formation of these compounds were due to addition of 1- and 2-tetralyl and 1- and 2-hydronaphthyl radicals to 1,2-dihydronaphthalene. The presence of C20 compounds as only minor reaction products suggested that termination reactions involving tetralyl radicals were primarily disproportionation reactions (for example, reaction 6), in contrast to thermal cracking of toluene and ethylbenzene' in which radical combination reactions led to the formation of double-ring compounds as major reaction products. Methane was the most abundant hydrocarbon gas. A typical gas composition (mole percent and hydrogen-free basis) was methane 52 % , ethane 22%, ethylene 15%, propylene 85% ,and propane 3 5%. The presence of methane as the major product gas was also reported by Hooper et al.23 for thermal cracking of tetralin under similar conditions. Dealkylation of l-methylindan to indanZ3and indene17 has been suggested as possible pathways for methane formation. 1,2-Dihydronaphthalenewas probably present as a trace impurity in the feed but its peak was overlapped by the tetralin peak in the chromatographic analysis. Hence it was not possible to quantify this compound in the feed or liquid products. Formation of dehydrogenated compounds is expected to be quite low under the relatively high pressures employed in this study. For example, de Vlieger et 01.26 reported that 1,2-dihydronaphthalene was absent in the liquid products from thermal cracking of tetralin at 400 "C and 10MPa while the selectivity for naphthalene was lower than the selectivity for l-methylindan. Hooper et al.23 also reported naphthalene as the only dehydrogenated product in thermal cracking of tetralin at 400450 "C and estimated pressures of 4 MPa a t 400 OC to 10 MPa at 450 "C. They also reported significantly lower
Energy & Fuels, Vol. 7,No. 6,1993 963 selectivities for naphthalene compared with l-methylindan. Due to the limitation of the chromatographicanalysis, formation of 1,2-dihydronaphthalene could not be confirmed or ruled out under the conditions employed in this study. The presence of hydrogenolysis products (n-butylbenzene and other monosubstituted benzenes) would suggest the presence of hydrogen atoms produced from decomposition of tetralyl radicals (reactions 5a and 5b). Hydrogenolysis of the saturated ring of tetralin (reaction 10) would lead to the formation of primary n-butylbenzene radicals which could isomerize to secondary radicals (reaction 11). The primary and various secondary n-butylbenzene radicals could either abstract a hydrogen from tetralin to give n-butylbenzene or can decompose by 0-scission to give C1 to C3 and monosubstituted benzene products (reaction 11-15).
- r' +
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(14)
An alternative mechanism for the hydrogenolysis of tetralin is via the radical hydrogen-transfer (RHT) mechanism. This mechanism has been proposed by McMillen and Malhotram2to be responsible for bond scission during coal pyrolysis and involves hydrogen transfer from a cyclohexadienyl radical. RHT has also been suggested to be the likely mechanism for cleavage of aryl-alkyl bonds in pyrolysis of polycyclic a l k y l a r e n e ~ . In ~ ~tetralin ~ pyrolysis,1-and 2-hydronaphthyl radicals can be produced by RRD involving 1,2-dihydronaphthalene (reaction 71, by hydrogen abstraction from 1,2-dihydronaphthalene, (30) McMillen,D.F.;Malhotra,R.;Hum,G.P.;Chang,S.-J. HydrogenTransfer Promoted Bond ScissionInitiated by Coal Fragments. Energy Fuels 1987,1, 193-198. (31) McMillen,D. F.;Malhotra,R.;Chang, S.-J.; Ogier, W. C.;Nigenda, S. E.; Fleming,R. H. Mechanismsof HydrogenTransfer and Bond Scission of Strongly Bonded Coal Structures in Donor-Solvent System. Fuel 1987,66,-1611-1620. (32) McMillen,D.F.;Malhotra,R.;Nigenda,S.E.TheCaseforInduced Bond Scission during Coal Pyrolysis. Fuel 1989,68,380-386. (33) Savage,P. E.; Jacobs, G. E.; Javanmardian, M. Autocatalysis and Awl-AlkvlBondCleavaae in l-DodwlpyrenePpolysis. Ind. Eng. Chem. - __ . . Res. 1989, 28, 645-654.(34)Smith, C. M.; Savage,P. E. Reactionsof PolycyclicAlkylaromatics. 1. Pathwavs. Kinetics. and Mechanisms for l-Dodecylpyrene Pyrolysis. - __ . Ind. Eng. chem. Res. '1991,30, 331-339. (35) Smith, C. M.; Savage,P. E. Readionsof Polycyclic Alkylaromatics. 4. Hydrogenolysis Mechanism in l-AlkylpyrenePyrolysis. Energy Fuels 1992,6,190-202. (36) Smith, C. M.; Savage, P. E. Hydrogenolysis Mechanism for Polycyclic Alkylarenes. Prepr.Prepn.-Am. Chem. Soc., Diu. Fuel Chem. 1992, 37(2), 937-946.
2
Khorasheh and Gray
Energy &Fuels, Vol. 7, No. 6, 1993
964
, i= 027
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Figure 3. Distributionsof productsin thermal cracking of R - C I ~ in tetralin.
and by addition of hydrogen atom to naphthalene (reaction 16). The RHT path for hydrogenolysis of tetralin involving
hydronaphthyl radicals is given by reaction 17.
"+m-
+
Q-•
(17)
The formation of hydrogenated products (propylbenzene and propane) suggested hydrogen transfer to the olefinic products. This could involve addition of free hydrogen atoms (reaction 18, for example) or by RRD involvingtetralin or 1,2-dihydronaphthalene (reaction 19, for example).
Hydrogen transfer from a radical to an olefin3' or to an aromatic s y ~ t e m ~is~ 1an3 ~alternative mechanism for hydrogen transfer from tetralyl or hydronaphthyl radicals to olefins (reaction 20, for example).
Thermal Cracking of n-Cle in Tetralin. Typical product distributions from thermal cracking of n-Cl6 in tetralin are presented in Figure 3. The major n-(&-derived products were complete series of n-alkanes in the C1 to c14 range, a-olefins in the C2 to c15 range, and cis- and trans2-olefins in the C4 to C15 range. Also present in the liquid products were two series of alkyltetralins with the alkyl chain containing 2-15 carbon atoms. These two series (37) Metzger,J. 0.Metathesis of Alkyl Radicals and Alkenes-a Novel Elementary Reaction of Alkyl Radicals. Angew. Chem., Znt. Ed. Engl. 1986, % ( I ) , 80-81. (38) Billmera, R.;Griffith, L. L.;Stein, S.E. HydrogenTransfer between Anthracene Structures. J. Phys. Chem. 1986, 90,517-523. (39) Billmera, R.;Brown, R. L.; Stein, S.E. Hydrogen Transfer from 9J0-Dihydrophenanthrene to Anthracene. Znt. J. Chem. Kinet. 1989. 21,375-386.
were 1-alkyltetralins (major series) and 2-alkyltetralins (minor series). Tetralin-derived products were qualitatively similar to those obtained in thermal cracking of tetralin by itself. The molar ratios of benzene and monosubstituted benzenes to n-butylbenzenes in the liquid products were also quite similar to those reported in the previous section. However, selectivities for naphthalene E4.21 slightly increased while selectivities for 1-methylindan L22.01 and the unidentified product assigned to indan 10.441decreased in the presence of n-Cl6. Some adjustments were necessary in estimating the product selectivities from thermal cracking of n-Cu as C1 to C3 compounds were derived from decomposition of both tetralin and n-Cls. Selectivities for these gases, in particular for the saturated gases, were higher than expected selectivitiesfrom thermal cracking Of n-C16. A lower bound on n-(&derived methane selectivity was estimated based on the sum of selectivities for l - c ~ cis, and trum-2-C15's, and a-and 8-pentadecyltetralins which were formed from the addition of 1-and 2-tetralyl radicals, respectively, to the terminal carbon of l-Cl5. One mole of each of these products indicates the formation of 1 mol of methane. The molar selectivities for "excessnmethane from tetralin were estimated as the difference between total and n-Cl6derived methane selectivities. Ethane and propane selectivities were also adjusted to account for excess gases produced from the decomposition of tetralin. The ratios of excess ethane and excess propane to excess methane were estimated to be 0.6 (C2's to methane ratio) and 0.25 ((23's to methane ratio), respectively, based on gas compositions from thermal cracking of tetralin by itself. Figure 3 illustrates some typical characteristics of the product distributions from thermal cracking of n-C16 in tetralin. The equimolar distribution of n-alkanes indicated that alkyl radicals generated from decomposition of parent hexadecyl radicals were stabilized much faster by abstracting a hydrogen from tetralin to give the corresponding n-alkanes than by decomposing (via &scission) to give a smaller radical and an a-olefin. Fast hydrogen abstraction from tetralin resulted in total product selectivities close to 200 mol per 100 mol of n-C16 decomposed. Hydrogen abstraction from tetralin could involve hydrogens in both a- and 8-positions (reactions 21 and 22, respectively) though the former is more favorable since the resulting 1-tetralyl radicals are resonance stabilized.
Addition of 1- and 2-tetralyl radicals to a-olefins (reactions 23 and 24, respectively, involving 1-pentene,
for example) led to the formation of alkyltetralin radicals which subsequently abstracted a hydrogen to give 1-and 2-alkyltetralins.
Thermal Cracking of n-Hexadecane in Tetralin
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Energy & Fuels, Vol. 7, No. 6, 1993 965
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0
16
Carbon number
Figure 4. Distributions of products in thermal cracking of 0.05 mole fraction n-Clein tetralin at 430"Cas a function of conversion.
C E
8
4
z
1 2 1 6
Converr1on
T(.C)
400 e 410
0
;:I 1 10
CIS
0
A A 0
420 430 440
450 460
& trans 2-0i.tinr
Figure 6. Overall product selectivities in thermal cracking of 0.03 mole fraction n-Cle in tetralin. n-C,,
x
'
0
2
4
6
8
101214
Cwbon number
0
2
mol i r o c l o n s O 03 conv.rs1on i ('C)
4
6
8
IO I 2 1 4 16 I O 1 2 I 4
Corbon number
16 18 20 22 24 26
Carbon number
Figure 5. Distributions of products in thermal cracking of 0.03 mole fraction n-Cle in tetralin as a function of temperature. Product distributions presented in Figure 3 also indicated that the total molar selectivity for n-alkanes from decomposition of n-Cl6 was in excess of 100 mol per 100 mol of n-C16 decomposed. This result suggested that a-olefins generated from the decomposition of were subsequently reduced to corresponding alkyl radicals. Hydrogen transfer to a-olefins could involve the following mechanisms: addition of hydrogen atoms generated from decomposition of tetralyl and hydronaphthyl radicals (reactions 5 and 9) to a-olefins to give alkyl radicals (reactions similar to reaction 18), RRD involving tetralin and 1,2-dihydronaphthalene (reaction 19, for example), and by hydrogen transfer involving tetralyl and hydronaphthyl radicals (reaction 20, for example). The resulting n-alkyl radicals subsequently abstracted hydrogen to give the corresponding n-alkanes. Typical product distributions as a function of d l 6 conversion are presented in Figure 4 for 0.05 initial n-Cl6 mole fraction. Molar selectivities for a-olefins decreased with increasing conversion as a-olefins were consumed in secondary reactions. These secondary reactions included isomerization to cis- and trans-2-olefins, addition reactions involving tetralyl radicals leading to the formation of alkyltetralins, and reduction to corresponding alkyl radicals. Addition reactions either involving H or tetralyl radicals have low activation energies and are favored at low reaction temperatures. Typical product distributions as a function of temperature are presented in Figure 5 for experiments with same initial mol fraction of n-Cl6 and relatively similar conversions. Molar selectivities for n-alkanes and alkyltetralins increased while molar selectivities for a-olefins decreased with decreasing reaction temperature. Product distributions presented in Figures 4 and 5 indicate that selectivities as a function of temperature or conversion do not shift with carbon
number; rather, they remain nearlyequimolar as the entire class of compounds is affected. The overall product selectivities from thermal cracking of 0.03 mole fraction n-Cl6 in tetralin are presented in Figure 6. Similar trends were observed for 0.01 and 0.05 mole fraction experiments. In all cases, total molar selectivites for a-olefins decreased with decreasing reaction temperature and with increasing conversion as a-olefins were converted to secondary products. The consumption of a-olefins was quite significant even at low n-Cl6 conversions. In the case of 0.03 mole fraction experiments, for example, total molar selectivities for a-olefins at 410 "C were 15.7 and 12.0 mol per 100 mol of n-& decomposed for 5.8 and 8.6% n-Cl6 conversion, respectively. The consumption of a-olefins at agiven n-Cl6 conversion was more pronounced at lower reaction temperatures. Total molar selectivities for n-alkanes were in excess of 100 mol per 100 mol of n-C16 decomposed and were found to increase with decreasing temperkture (indicating low activation energies for olefin reduction reactions) and increasing n-C16 conversion (higher a-olefin concentration). Depending on the reaction temperature, however, molar selectivites for other secondary products (cisand trans-2-olefins and alkyltetralins) either increased, decreased, or went through a maximum with increasing conversion. In all cases, however, selectivities for alkyltetralins were greater than selectivities for additional n-alkanes. The molar ratio of alkyltetralins to additional n-alkanes in the products varied between 1.5 and 15 depending on the initial mole fraction, reaction temperature, and n-C16 conversion. This ratio increased with increasing n-Cl6 mole fraction and increasing reaction temperature, and decreased with increasing n-Cl6 conversion. This observation suggests that, under conditions employed in this study, addition of solvent radicals to a-olefins is more favorable than a-olefins reduction to alkanes. A t zero conversion of n-C16, the selectivities for alkyltetralins and internal olefins would be zero, because these are secondary products from a-olefins. The selectivity for these products must increase very rapidly with conversion to achieve values illustrated in Figures 6. In the case of n-alkanes, however, trend lines for selectivities as a function of n-Cl6 conversion all converge to 100 mol per 100 mol of n-C16 decomposed when extrapolated to zero conversion.
966 Energy & Fuels, Vol. 7, No. 6, 1993
The observed maxima in selectivities for internal olefins and alkyltetralins with increasing n-Cl6 conversion could be explained in terms of competing reactions leading to the formation of these compounds. Consider, for example, reactions involving 1-butene as a representative a-olefin (reactions5a, 26-29) where reaction 26 is a possible reaction
n-C16 DHN DHN
Khorasheh and Gray
- -+ + R;
+
R. 3 1-T'or2-T'
DHN
T
1-T' or 2-T'
---
Primary R' (or n-Ci6)
+
X
n-C16
"-Cia
Ri
R'
+ +
X X
2-T
+
a-oldn
T
+
a-oldn
a-olefn
---+
--
H'
+N
H'
+
DHN
H'
+
o-olefin
N
+ H' + H'
1-T'or2-T'
-
1-T'or2-T'
X
24kyltetralin radical (2-T-R)
1-HN'or 2-HN'
1-HN' or 2-HN'
T
1-HN'or2-HN'
l-alkyltetralin radical (1-T-R)
DHN
1-HN' or 2-HN'
+
+
R
a-ole6n
a-oldn
+ o-oldn + a-oldn +X x-x
DHN
1-T'or2-T
+
X-H
1-T' or 2-T'
n-Ci6
a-oldn
X-H
DHN
+ +
1-T'
+
1-T'or2-T
1-HN' or 2-HN'
Secondary R' (or n-Cis)
+
X-H
R'
+ +
N
+
R'
DHN
+
R'
l-HN'or2-HN'
1-T'or2-T
+
R'
+R
Radical Disproportionation, for example:
1-T'
+ 2-HN'
-+ T
N
X = H', R ,n-C 16', 1-T, 2-T', 1-HN', 2-HN', 1-T-R', 2-T-R
Figure 7. Simplified reaction mechanism for thermal cracking leading to the isomerization of a-olefins to cis- and trans2-olefins: The rate of conversion of a-olefins to secondary
c
L-
-w
*w
H .b.
(30)
products (reactions 25-29) is zero a t zero n-Cl6 conversion. Thetate of each of these reactions is proportional to the concentration of a-olefins and increases with increasing n-C16 conversion as a-olefins are produced from n-C16 decomposition. The fraction of a-olefins converted to alkyltetralins, internal olefins, and additional alkanes depends on the relative rates of reactions 25,26, and 2729, respectively. The occurrence of maxima in selectivities for alkyltetralins and internal olefins suggests that the concentration of tetralyl radicals decrease while the concentration of H' and hydronaphthyl radicals increases with increasing n-C16 conversion. The above discussion on thermal cracking of n-Cl6 in tetralin is summarized in terms of a simplified mechanism (Figure 7) with solvent interactions. The free-radical mechanism is initiated primarily by the cleavage of C-C bonds of n-Cle (reaction 31) and by RRD involving 1,2dihydronaphthalene and tetralin (reactions 7 and 8). Reactions involving tetralin (for example, reactions 1and 2) could also contribute to some extent as a source of initiation for free radicals. Alkyl radicals generated from decomposition of parent hexadecyl radicals (reaction 34), could isomerize via 1-4 and 1-5 internal hydrogen abstraction, reaction 32, and decompose to an a-olefin and a smaller radical, reaction 35. Radicals could participate in hydrogen abstraction from n-Cu, reaction 33, from tetralin, reaction 36, or from 1,2-dihydronaphthalene, reaction 37. Hydrogen abstraction from tetralin involves hydrogens in the a- and 8-positions of the saturated ring. The resulting tetralyl radicals could stabilize by hydrogen
of
in tetralin with solvent interactions.
abstraction from n-Cl6, tetralin, and 1,2-dihydronaphthalene, and participate in radical addition reactions with a-olefins, reactions 38 and 39. The resulting alkyltetralin radicals subsequently abstract a hydrogen to give the correspondingalkyltetralin. Decompositionof tetralyl and hydronaphthyl radicals, reactions 40 and 41, respectively, is a source for formation of hydrogen atoms. Hydronaphthy1 radicals are produced by RRD involving 1,2-dihydronaphthalene (reactions 8 and 47), by hydrogen abstraction from 1,2-dihydronaphthalene, and by addition of hydrogen atom to naphthalene. Hydrogen atoms could abstract a hydrogen or participate in addition reactions with naphthalene, 1,2-dihydronaphthalene,and a-olefins, reactions 42-44, respectively. a-Olefins could also be reduced to corresponding alkyl radicals by hydrogen transfer involving hydronaphthyl and tetralyl radicals, reactions 45 and 46, respectively, and by RRD involving 1,2-dihydronaphthalene and tetralin, reactions 47 and 48, respectively. Termination reactions include assorted radical combinations (reaction 49) or disproportionation involving tetralyl and hydronaphthyl radicals (reaction 50). Overall Kinetics. The Arrhenius plot of the apparent fiist-order rate constants for the overall conversionOf n-Cl6 in tetralin is presented in Figure 8. Activation energy was approximately 60.0 kcal/mol. Similar observations can be made with regards to the first-order rate constants for thermal cracking of n-Cl6 in tetralin as those described for thermal cracking of n-Cl6 in toluene and ethylben~ e n e . ~ The * ~ * first-order rate constants for overall conversion Of n-Cl6in the presence of tetralin were significantly lower than corresponding values for thermal cracking of pure n-Cl6' or n-Cl6 in benzene7 at the same pressure, because preferential hydrogen abstraction from the solvent
Thermal Cracking of n-Hexadecane in Tetralin
Energy & Fuels, Vol. 7, No. 6, 1993 967
T(.C)
loD
460
450
440
430
420
410
400
:
'\
-.........................................
.....................................
i
\
10-2
1
......................................................................................................................................................... 410
0
0
v 0.01
-
0
v 0.01 0
0 0.03
1 o-'
.F).....
0.03
D 0.05
7 .I
0
1.36
1.38
1.40
1.42
1.44
1/T x 1000 (K
1.46
1.48
-l)
Figure 8. Arrhenius plot of apparent first-order rate constants for overall conversion of in thermal cracking of n-Cl6 in tetralin.
inhibited the chain mechanism for conversion of n-C16. These first-order rate constants did not show any dependence on the initial mole fraction of n-C16. Product inhibition was also observed in thermal cracking of n-Cl6 in tetralin as the first-order rate constant for overall conversion of n-Cl6 decreased with increasing n-C16 conversion (Figure 9).
Conclusions The following conclusions can be made from the results of this study: 1. The fast rate of hydrogen abstraction from tetralin resulted in an equimolar distribution of n-alkanes from decomposition of n-CI6 as well as total n-Cl6-derived product selectivities close to 200 mol per 100 mol of n-Cu decomposed. 2. The preferential hydrogen abstraction from tetralin rather than n-C16, inhibited the chain mechanism for conversion of n-C16. 3. Hydrogen abstraction from tetralin involved hydrogens in both a-and @-positionsof the saturated ring leading to the formation of 1- and 2-tetralyl radicals. These
2
4
6
8
10
12
14
16
7; Convrnlon
Figure 9. Apparent first-order rate constants for thermal cracking of n-Cle in tetralin.
radicals participated in addition reactions with a-olefins leading to the formation of 1-and 2-alkyltetralins as major reaction products. 4. a-Olefins were also reduced to alkyl radicals which subsequently abstracted a hydrogen to give the corresponding n-alkanes. Possible mechanisms for olefin reduction include addition of hydrogen atoms generated from decompositionof tetralyl and hydronaphthyl radicals, hydrogen transfer from tetralyl and hydronaphthyl radicals, and RRD involving tetralin and 1,2-dihydronaphthalene. Total n-alkane selectivities therefore, were in excess of 100 mol per 100 mol of n-Cl6 decomposed, and increased with increasing conversion. 5. Addition of tetralyl radicals to a-olefins was the dominant secondary reaction involving a-olefins. Depending on the reaction conditions, the molar ratio of alkyltetralins to additional n-alkanes in the products varied between 1.5 and 15.
Acknowledgment. We thank the reviewers for their helpful comments and insight. Financial support was provided by Alberta Oil Sands Technology and Research Authority (AOSTRA) under agreements 521 and 781, and by Esso Petroleum Canada under University Research Grants Program.
