Energy & Fuels 1995,9, 1090-1096
1090
Pathways for the Deoxygenation of Triglycerides to Aliphatic Hydrocarbons over Activated Alumina Enrico Vonghia, David G . B. Boocock,* Samir K. Konar, and Anna Leung Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario, Canada M5S 1A4 Received June 20, 1995. Revised Manuscript Received September 1 I, 1995@
The functional group pathways for the high-yield conversion of triglycerides to aliphatic hydrocarbons (mainly monoalkenes)when passed over activated alumina at 450 "C (weight hourly space velocity (WHSV) 0.28-0.46 h-l) were studied by using model compounds believed to be intermediates in the process. These compounds included methyl ketones, dodecyl aldehyde, and dodecanol. Previous results had been obtained on carboxylic acids and symmetrical ketones. It is concluded that the triglycerides can split out carboxylic acids by j3-elimination or yield alkenes by a y-hydrogen transfer mechanism. The carboxylic acids form symmetric ketones which preferably undergo a further y-hydrogen transfer to produce monoalkenes and methyl ketones. An unknown reductive mechanism followed by dehydration produces monoalkenes from the methyl ketones. The latter can also undergo the y-hydrogen rearrangement. It is proposed that the methyl ketone can also isomerize to aldehydes, which then undergo an oxidation producing carboxylic acids. This may involve a disproportionation and be coupled with the reduction mentioned previously. A cycling of the carboxylic acids through the ketone route is believed to be the source of monoalkenes having one more carbon atom than the methyl ketones. The relevance of this study to petroleum biogenesis is obvious.
Introduction Recently, there has been an interest in the catalytic conversion of triglycerides, which include vegetable oils, into liquid hydrocarbon fuels.'I2 Two conversion methods among the most prominent are low-temperature, liquid-phase catalytic processes such as transesterification t o methyl or ethyl esters, and high-temperature pyrolytic processes involving solid catalysts. l'ransesterification can easily and conveniently convert a variety of oils to almost quantitative yields of acceptable diesellike fuel a t room temperature in a matter of hours.2 Pyrolytic processing of numerous plant extracts and vegetable oils may also produce hydrocarbon liquids possessing properties similar t o diesel fuel and gasoline, but this requires the presence of cracking catalyst for there to be significant c o n ~ e r s i o n . ~We ? ~have shown that when the catalyst is replaced with glass beads, little, if any, conversion OCCUTS.~ It is reported that higher temperatures are required for the noncatalytic pyrolysis of ester^.^ F'yrolytic conversion technologies may be further divided into hydrogenation and non-hydrogenation processes. Hydrogenation was employed by Feng and coworkers6to convert the triglyceride content of depitched tall oil from the kraft pulping industry into straight-
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, October 15, 1995. (1)Rao, K. V. C. U S . Patent 4,102,938, 1978. (2) Shay, E. G. Biomass Bioenergy 1993,4 , 227-242. (3) Boocock, D. G . B.; Konar, S. K.; Mackay, A,, Cheung, P. T. C.; Liu, J. In Aduances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Chapman and Hall: Glasgow, U.K. 1994, pp 10001015. (4)Sharma, R. K.; Bakhshi, N. N. Can. J. Chem. Eng. 1991,69, 1071-1081. (5) DePuy, C. H.; King, R. W. Chem. Reu. 1960,60, 431-457. (6) Feng, Y.; Wong, A,; Monnier, J. Proc. First Biomass Conf. Am.; Burlington, VT 1993,863-875.
chain alkanes. In this patented technology, the biomass oil was reduced with gaseous hydrogen over a conventional petroleum hydroprocessing catalyst a t elevated temperatures and pressures. The procedure effectively removed 99.7% of the oxygen and all of the acids previously present in the tall oil feedstock. Both the cyclic hydrocarbon and aromatic contents of the product were much lower than those in conventional diesel fuels. Non-hydrogenation processes have employed numerous catalysts t o dehydrate and decarboxylate biomass oils. Chow7 cracked a range of vegetable oils, their derivatives, and natural and processed rubbers over both aluminosilicate zeolites and activated alumina at pressures between 1 and 10 atm and temperatures of 420-550 "C. Among the products obtained were gaseous and liquid hydrocarbons, coke, and water. However, suitable fuel fractions could only be obtained by further fractional distillation. The aluminosilicate zeolite HZSM-5 was also used in numerous microscale studies by Sharma and Bakhshi4 to completely convert various vegetable oils, plant extracts, and natural rubbers to gasoline boiling range hydrocarbons, water, and light gases including oxides of carbon a t 400-500 "C temperatures and 1-2.5 h-' WHSVs. For canola oil in particular, 60-95% was converted into hydrocarbons in the gasoline boiling range, of which 60-70% was aromatic hydrocarbons and included ethylbenzene, benzene, toluene, and xylenes. Gaseous product yields of 8-10% were obtained, and were predominantly parafflniC.8 The complex aromatic and cyclic product distributions resulting from biomass pyrolysis over zeolites complicate (7) Chow, Peng W. US.Patent. 5,233,109, 1993. ( 8 ) Prasad, Y. S.; Bakhshi, N. N.; Mathews, J. F.; Eager, R. L. Can. J . Chem. Eng. 1986,64, 278-284.
0887-0624/95/2509-1090$09.00/00 1995 American Chemical Society
Deoxygenation of Triglycerides to Aliphatic Hydrocarbons a determination of the conversion pathways involved. Activated alumina, on the other hand, is ideal for studying primary decomposition pathways because it deoxygenates the substrate solely to acyclic alkenes and some alkanes. Early studies by Demorest et ala9decomposed acetone and butyric acid over activated alumina at 420 and 478 "C, respectively. Total liquid yields were low for acetone (42.8%)but higher for butyric acid (74.8%), although ketonic compounds dominated both products. h j o s and co-workers1°passed crude soybean oil vapor over activated alumina in a tubular glass reactor a t temperatures of 300, 400, and 500 "C and obtained relatively low ( 5 5 4 8 % ) total liquid yields. Roughly half of this product consisted of oxygenated materials such as carboxylic acids, aldehydes, and ketones, while the rest (29-30%) were hydrocarbons similar in nature to diesel fuel. "he low yields and highly oxygenated products of previous pyrolyses using activated alumina were probably due to high feed rates and incomplete catalyst demoisturization. Recently, however, another pro~ e s s , ~for J l the deoxygenation of sewage sludge lipids also efficiently converted triglycerides such as vegetable oils in high (69-73%) yields to a liquid mixture of straight-chain alkanes and alkenes. In all cases, the liquid product was highly mobile and of low viscosity, and from 13C NMR and IR spectra, contained primarily monoalkenes, including some with thermodynamically unfavorable terminal double bonds. A notable absence of carbonyl groups was also observed in the product IR spectra. Several model compounds, including triolein and trilaurin as well as canola oil and coconut oil, have already been pyrolyzed to investigate the triglyceride conversion pathways during this catalytic p r o ~ e s s .A ~ y-hydrogen transfer mechanism was proposed to explain the high quantities of terminally-bonded monoalkenes formed, while detailed pyrolysis studies by Leung of this group12have suggested that a /3-elimination mechanism also occurs, which initially cleaves carboxylic acid from the triglyceride. Other intermediates observed in these studies were symmetrical and methyl ketones. The aim of this paper is to summarize the functional group pathways of triglyceride decomposition over activated alumina which have so far been identified. Wherever possible, proposals for specific mechanisms are also made, based on the observed pyrolysates of a number of model compounds which include 2-undecanone, dodecyl aldehyde, and dodecanol.
Experimental Section Materials and Equipment. Crude canola oil was supplied by Procter and Gamble Inc., Toronto, Ontario, Canada. Commercial-grade acetone, 92% dodecyl aldehyde, 96% nonanoic acid, 90% oleic acid, and 99% 2-undecanone were supplied by the Aldrich Chemical Co., Milwaukee, WI. Glacial acetic acid was obtained from BDH Inc., Toronto, Ontario, Canada. Dodecanol was provided by J. T. Baker Chemical Co., Phillipsburg, N. J. The Alcan AA 200 activated alumina used was (9)Demorest, M.; Mooberry, D.; Danforth, J. D. Ind. Eng. Chem. 1961,43,2569-72. (10)&os, J. R. S. D.; Lan, Y. L.; Frety, R. Bol. Tec. PETROBRAS 1981.24. 139. ----I
- - 7
(11)Konar, S. K.; Boocock, D. G. B.; Mao, V.; Liu, J. Fuel 1994, 73,
642-646. (12)Leung, A.; Boocock, D. G. B.; Konar, S. K. Energy Fuels, in press.
Energy &Fuels, Vol. 9, No. 6, 1995 1091
Figure 1. Experimental setup: (1)316 stainless steel feed preheater tube (1.3 cm i.d. x 50 cm length); (2) block heater containing a 316 stainless steel fmed bed reactor tube (2.5 cm x 46 cm length); (3)catalyst bed; (4) Type J (irodconstantan) thermocouple probe; ( 5 ) Type J (irodconstantan) thermocouple with temperature controller; (6) syringe pump; (7) condenser; (8) receiving flask; (9) gas trap; (10) gas collection vessel; and ( 11)nitrogen cylinder. supplied by Alcan Chemicals, Brockville, Ontario, Canada. The catalyst's BET surface area is 270-290 m2/g, and its pore volume is 40 cm3/g. The pore distribution is binodal, with 66% of pore volume in pores less than 30 A radius and the rest in larger pores. Its bulk density is 0.75 g/cm3. Infrared spectra were recorded on a Perkin-Elmer model 598 IR spectrophotometer using the thin film method. Liquid samples were analyzed using a Hewlett Packard 5880A series gas chromatograph (GC) equipped with a flame ionization detector and a DB17 (30 m x 0.53 mm i.d.1 fused silica capillary column. The operating parameters were as follows: detector temperature 225 "C; injector temperature 225 "C; temperature program, 5 min at 50 "C; heated a t a rate of 5 "C/min to 210 "C; held for 23 min. The gas flow rate was 59.6 mumin, which included both the carrier gas He and the makeup gas. Gas analyses were performed on a similar GC equipped with a flame ionization detector and a 316 stainless steel column (2.44 m x 3.18 mm i.d.) with a Poropak type QS packing material. The operating conditions were as follows: detector temperature, 120 "C; injector temperature, 120 "C; temperature program, 1 min at 50 "C; heated at a rate of 30 "C/min to 150 "C; hold for 20 min. Carrier gas (Ar)flow rate was 40.05 mumin. Gas chromatography-mass spectrometric (GC-MS) analyses were done on a Hewlett Packard 5890 GC equipped with a J&W Scientific DB-5 capillary column (30 m x 0.250 mm id.) and using helium carrier gas at a volumetric flow rate of 1 mumin. The linear flow velocity was 32 c d s . GC-MS was performed by the Department of Chemistry, University of Toronto, courtesy of Professor T o p Tidwell and Dr. Alex Young. Analytical hydrocarbon standards were supplied by Polyscience Corp., IL. Carbon and hydrogen elemental analyses were performed by Guelph Chemical Laboratories, Guelph, Ontario, Canada. Pyrolysis Apparatus and Procedure. The pyrolysis unit consisted of an insulated 316 stainless steel preheater tube (1.3 cm i.d. x 50 cm length) which extended 1 in. into a 316 stainless steel fixed bed tubular reactor (2.5 cm i.d. x 46 cm length), which was heated by a cylindrical block heater. Two type J (iron-constantan) thermocouple probes were used to both monitor the internal catalyst bed temperature and maintain a consistent reactor wall temperature in combination with a temperature controller. A syringe pump, condenser, vacuum adapter, receiving flask, nitrogen cylinder, and gas collection system were connected as shown in Figure 1. The reactor midsection was packed with 40 g of activated alumina, which was held in place by a circular stainless steel screen. The preheater and reactor were operated at 180-190 and 450 "C, respectively. The entire process remained at normal atmospheric pressure throughout the run. Prior to a run, fresh catalyst was demoisturized at 450 "C for a 3 h period by intermittently passing nitrogen over the
Vonghia et al.
1092 Energy & Fuels, Vol. 9, No. 6, 1995 Table 1. Flow Rates, Liquid Yields, and Product Masses substratea 2-undec- 2-undecanonel dodecyl aldehyde anone acetone flow rate (mumin) 0.34 0.34 0.22 WHSV (h-l) 0.42 0.42 0.28 substrate mass (g) 81.8 59.7 32.3 organic liquid product (g) 66.0 43.4 23.9 organic liquid product yield 80.7 72.7 74.0 total liquid product (g) 70.0 49.0 24.8 total gas product (g) 4.4 1.8 Ob total coke product (g) 4.8 4.7 2.7 weight percent recovered 96.8 93.0 85.1
" For canola oil substrate and its results, see ref 3. Possible leakage.
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catalyst bed. High demoisturization was necessary in order to activate the catalyst. One hour of treatment was inadequate, and 2 h was only barely adequate to activate the catalyst. Immediately before commencing liquefaction, a nitrogen environment was established in the pyrolytic system. A syringe pump (Sage Instruments (subsidiary of Orion Research Inc.), Model 355, Sage Instruments Inc., White Plains, New York) was used to inject liquid substrate (ranging between 30 and 100 mL volumes) from a 100 mL syringe into the preheater. From previous ~ t u d i e s , ~the J l optimum feed rate which gave the highest organic liquid yield was 0.34 mL1 min, which is equivalent to a 0.46 h-' weight hourly space velocity (WHSV)for crude canola oil. Consequently, this was also the rate employed for most of these studies, although for certain runs, this feed rate was altered. In addition, the reaction time was chosen such that catalyst deactivation was minimal. Reactant amounts, their flow rates and corresponding WHSVs are all presented in Table 1. The preheater was angled so the injected substrate proceeded by gravity flow into the reactor. Substrates which were solids at room temperature (i.e., dodecanol) were melted and maintained as liquids by wrapping both the syringe and the preheater entrance area with heating tapes during injection. The pyrolyzed product exiting the reactor was cooled with a water condenser and collected in one or more 100 mL receiving flasks immersed in an ice bath. Noncondensable products passed through the gas trap into the brine solution and were measured by brine displacement into a 500 mL graduated cylinder. After all substrate was injected, an additional 8-15 min period allowed for the residual substrate to exit the reactor. The liquid product and syringe were then weighed to determine product yield, and the spent catalyst was also weighed t o obtain coke deposition quantities.
Results All initial liquid pyrolysis products consisted of a highly mobile upper organic layer, and a lower pyrolytic water layer. Most pyrolysates were light yellow, whereas the 2-undecanone and equimolar 2-undecanone/acetone pyrolysate fractions were a deep orange-red. Each product also possessed a distinct alkene aroma, and exhibited a blue-green fluorescence. For the pyrolysate of 1-dodecanol,a transparent, colorless organic product was observed. Flow rates and liquid product yields, as well as mass balances for the pyrolysis trials, showing liquid, noncondensable gas, and carbonaceous residue (coke) yields, are presented in Table 1. The noncondensable gas volumes were converted into mass quantities using a representative gas composition determined by GC analysis. The primary gaseous components were Cq hydrocarbons. The carbon and hydrogen elemental compositions for 2-undecanone and dodecyl aldehyde pyrolysis
Figure 2. GC spectrum of the 2-undecanone pyrolysis product.
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Figure 3. GC spectrum of the 2-undecanonelacetone pyrolysis product. Table 2. Elemental Analyses" substrate C(wt%) H(wt%) C+H(wt%) 2-undecanone 85.87 13.70 99.57 dodecyl aldehyde 86.31 13.97 100.28
Error: f0.5 wt % for carbon.
products are given in Table 2. These results indicate that very little of the product mass is not accounted for by carbon and hydrogen. The oxygen content, by difference, was therefore very low. The IR spectra of the equimolar 2-undecanone/acetone pyrolysis product and that of the 2-undecanone product were almost identical. GC spectra of the pyrolysis products of 2-undecanone and equimolar 2-undecanone/ acetone are shown in Figure 2 and Figure 3. Figure 4 depicts the GC spectrum of the dodecyl aldehyde pyrolysate. On each GC, the retention times of n-alkane and 2-undecanone standards are presented. In many cases, duplicate runs were made, and results were consistent to within 3% of each other.
*
Discussion Triglyceride Decomposition Pathways. Some of the conversion pathways of triglycerides such as canola oil and glycerol trilaurate over activated alumina have already been identified.3J2 An overall s u m m a r y of these routes is presented in Figure 5. Glycerol trilaurate was
Deoxygenation of Triglycerides to Aliphatic Hydrocarbons
Energy &Fuels, Vol. 9,No. 6, 1995 1093 CHr0 O-C-RI
: : :1
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Figure 6. 8-eliminationand y-hydrogen transfer on a glycerol
trilaurate molecule.
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Figure 4. GC spectrum of dodecyl aldehyde pyrolysis product. 0 CH+#(CHi)loCHi
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Figure 6. Triglyceride conversion pathways.
considered appropriate for pathway illustration because of the detail in which its decomposition over activated alumina has already been investigated. It was postulated that triglyceride deoxygenation occurred via two initial mechanisms. Each mechanism is believed t o occur through the bonding of a carbonyl oxygen to a Lewis acid site on the alumina catalyst. The first was that of y-hydrogen transfer on one or more of the triglyceride chains to directly produce alkenes. The second possible mechanism was /?-elimination(1,2elimination), which produced carboxylic acids and unsaturated glycol difatty acid esters (UGDEs). Both mechanisms may occur before or after the other, although only one &elimination per triglyceride molecule is possible, while alkenes may be produced by y-hydrogen transfer on all three triglyceride chains. Both mechanisms are depicted in Figure 6, occurring simultaneously on a triglyceride molecule. The relative importance of both mechanisms in triglyceride deoxygenation may be determined from the product distributions of lauric acid, diundecyl ketone, and glycerol trilaurate. Pyrolysis of glycerol trilaurate3 yielded a majority of C10 alkenes, suggesting that
y -hydrogen transfer was a dominant conversion mechanism. This route was also the most reasonable in explaining the thermodynamically unfavorable terminally double-bonded alkenes observed in triolein, canola oil, trilaurin, and coconut oil pyrolysate^.^ However, earlier pyrolysis studies of glycerol triacetate (triacetin), which has no y-hydrogens, indicated that P-elimination could also occur.13 Triacetin would be formed by y-hydrogen transfer on all triglyceride chains and rearrangement of the resulting enols but was discounted as a major reaction intermediate because the acetic acid and both possible enediol diesters produced by its pyrolysis were not present in the highly deoxygenated triglyceride pyrolysates. When triglycerides are pyrolyzed under conditions of incomplete conversion, carboxylic acids and ketones are produced. The carboxylic acids formed by this route give products similar to their parent triglycerides when pyrolyzed under identical conditions.12 Evidence for the presence of the /?-elimination mechanism was also supported by the high fatty acid and UGDE yields obtained by Kitamura14from slow thermal noncatalytic decomposition of triglycerides. For unsaturated triglycerides such as triolein and refined canola oil, a more uniform hydrocarbon product distribution was ~ b t a i n e d .The ~ present studies using crude canola oil yielded similar pyrolysis products. The uniformity across the hydrocarbon mass range was attributed to additional y -hydrogen transfers occurring at /?-positionst o the double bonds. In addition to alkenes, some alkanes were also present in all pyrolysis products. Cleavage adjacent to the carbonyl group, and between the and y carbon atoms, followed by hydrogen abstraction are the proposed routes for the observed dominant C9 and C11 alkanes in the glycerol trilaurate pyrolysis p r ~ d u c t . ~ These pathways could also be concerted, and ethylene and carbon dioxide could be byproducts. Furthermore, the low amounts of alkanes in the pyrolysates of triglyceride intermediates such as 2-undecanone (Figure 2) reinforced the premise that alkanes were primarily formed by triglyceride chain cleavage. Carboxylic Acids. Previous studies concluded that the UGDEs and triacetin enols remaining after y-hydrogen transfer and /?-elimination on the triglyceride further converted, not t o ketone forms, but t o noncondensable gas phase product^.^ The carboxylic acids formed by the latter mechanism, however, underwent further reaction to ultimately form alkenes through a number of intermediates. The first of these were dialkyl ketones, formed by the condensation reaction of two (13)0,T. The Pyrolysis of Glycerol Triacetate. B.A.Sc. Thesis, Dept. of Chem. Eng, University of Toronto, 1991. (14)Kitamura, K. Bull. Chem. SOC.Jpn. 1971,44, 1606-1609.
1094 Energy & Fuels, Vol. 9, No. 6, 1995
fatty acids. In the case of glycerol trilaurate, its corresponding acid was lauric acid ((3121, which produced diundecyl ketone (C23). Pyrolysis studies with both lauric acid and diundecyl ketone12yielded identical product slates, showing that triglyceride conversion underwent this route almost completely and relatively rapidly. Kitamura14 also proposed ketone formation from triglycerides occurred via fatty acids. Symmetrical Ketones. When intermediate carboxylic acids are pyrolyzed or the substrate is a pure triglyceride, the ketones formed are symmetric. Following their formation from carboxylic acids, the symmetrical ketones underwent further y-hydrogen transfer to yield the enolic form of their methyl ketones as well as terminally double-bonded alkenes. The highly unstable enol product then underwent rapid keto-enol tautomerism, where rearrangement and migration of the double bond and a proton immediately formed the methyl ketone. The existence of this stage in triglyceride pyrolysis was supported by previous studied2 where pyrolysis of diheptyl ketone yielded considerable 2-nonanone under conditions which did not allow for complete conversion to alkenes. For glycerol trilaurate conversion, the corresponding methyl ketone formed from lauric acid was 2-tridecanone. Some evidence indicating that symmetrical ketones could also reduce directly to form alkene was given in studies where repyrolyzed 5-nonanone yielded nonenes.15 Pure acetic acid and acetone were also pyrolyzed under these conditions but yielded predominantly aromatic products such as alkylated benzenes and substituted cyclic alkyl dienes. However, these pyrolysis products were not believed to be representative of those formed when the percentages of both materials in the substrate were lower. Methyl Ketones. The present studies investigated the conversion of methyl ketone to alkenes by pyrolyzing 2-undecanone. The IR spectrum of the pyrolysate from this substrate showed bands at 1645,965, and 910 cm-l indicative of alkenes, and a very weak carbonyl peak at 1720 cm-', which revealed the product to be highly deoxygenated. Elemental analysis confirmed that the 2-undecanone pyrolysate consisted almost exclusively of carbon and hydrogen (only 0.43% of the product was oxygen, by difference)in a ratio characteristic of monoalkenes. Further GC analysis of this product exhibited peaks in the C6 to C9 hydrocarbon mass range and several peaks corresponding to C11 hydrocarbons. Surprisingly, two C12 hydrocarbon peaks were also present, although few compounds existed above this mass range. The same trends were observed in the GC spectrum of 2-tridecanone pyrolysis products.12 For 2-undecanone pyrolysis, the C6-C9, C11, and C12 mass fractions comprised 36.9, 41.0, and 11.4% of the total product, respectively. From the GC-MS total ion chromatogram and its associated mass spectra, the analyzed product hydrocarbons were all shown to be primarily alkenes or alkanes, with the major C9, C11, and C12 hydrocarbon peaks all identified as monoalkenes. Mass spectra for compounds lower than the C9 mass range were not obtained. The C11 mass fraction, in particular, con(15) Leung, A. Pathways for the Catalytic Conversion of Carboxylic Acids to Hydrocarbons over Activated Alumina, M.A.Sc. Thesis, Dept. of Chem. Eng., University of Toronto, 1994.
Vonghia et al.
tained several well-resolved alkene isomers. Residual higher molar weight compounds were oxygenated, consisting of materials such as straight-chain ketones and 2-methyl-2-phenyl-pentadecane. Both the clear absence of C10 hydrocarbon in the product from 2-undecanone and the fact that multiple C11 alkene isomers, but only two C12 alkene isomers were obtained made it clear that methyl ketone conversion proceeded through more than one pathway. One proposed route was that of y-hydrogen transfer on the methyl ketone, which would yield acetone and 1-octene. The presence of significant (9.1% of total) amounts of C8 hydrocarbons supports the existence of this conversion pathway. The other C6 and C7 hydrocarbons are probably formed by chain scission to yield the primary radicals, which rapidly extract hydrogen atoms from other molecules, while the C9 hydrocarbons are produced by an unknown route. It was initially believed that the reaction of acetone with these alkenes could account for those alkenes having the same number of carbon atoms as the methyl ketones. In the case of lauric acid and diundecyl ketone, it was believed that the recombination of decenes and acetone might be the route to C13 alkenes. Consequently, recombination of 1-decene and acetone was attempted by pyrolysis of an equimolar mixture under previous pyrolysis condit i o n ~ However, .~~ no reaction was observed between the two except for alkene isomerization. The failure of recombination was supported by the notable absence in the product of the branched dienes theoretically expected for this reaction. A second possible reaction the methyl ketones could undergo was that of reduction to an alcohol (via isomerizatioddisproportionation, as discussed later), then dehydration to mono-alkenes with the same carbon number. From the high quantities of undecenes obtained in 2-undecanone pyrolysis, it was clear that this was the preferred conversion pathway of the methyl ketone. The priority of this route was also supported by the reasonable amounts of pyrolytic water formed. This route yielded at least four distinct monoalkene isomers. However, it was difficult to establish from GC or GC-MS analysis whether these were all the isomers formed (five double bond isomers and four geometric isomers were possible). If the products had been dienes, an appropriate conversion mechanism could have been expressed by double bond migration in the enol form of the methyl ketone, followed by dehydration. The pyrolysis of an equimolar 2-undecanone/acetone mixture yielded further insight into the ketonic conversion. From the IR spectrum, extremely weak aromatic peaks were evident at 1600, 830, and 690 cm-'. The remainder of the spectrum was identical to those of triglyceride pyrolysates, with alkane C-H stretching bands in the 3000-2800 cm-l region, a weak carbonyl peak at 1720 cm-l, and alkene bands at 960 cm-l and 905 cm-'. The pyrolysate's GC spectrum (Figure 3) resembled that of pure 2-undecanone pyrolysate, with C6 to C9 hydrocarbons (44.7%), C11 monoalkenes (22.2%), and the two C12 monoalkenes (13.0%) all present. However, some higher molar mass compounds (C14-Cl9 mass range) were also evident in notable quantities, of which the most dominant was residual 2-undecanone. Product analysis showed that the addition of acetone enhanced certain conversion pathways of 2-undecanone.
Deoxygenation of Triglycerides to Aliphatic Hydrocarbons
Energy & Fuels, Vol. 9, No. 6, 1995 1096
In particular, the undecene/dodecene formation ratio changed significantly, decreasing to 2:l from an initial 4:l ratio for pure 2-undecanone pyrolysis. The dodecene content of the pyrolysis products from the mixed substrate and undecanone were 13.0 and 11.4%, respectively. The corresponding undecene contents were 22.2 and 41.0%. A possibility for acetone interaction with the methyl ketone is detailed in the next section. C 1 Alkene Formation. Although both the reductiorddehydration and y-hydrogen transfer pathways of the methyl ketone accounted for the majority of observed 2-undecanone pyrolysis products, the existence of products with one more carbon number than the substrate (C 1)remained unresolved. The significant 1 products was previously presence of several C observed in pyrolyzes of pentanoic, heptanoic and octanoic acids, and two C 1peaks in the same ratios as those in the 2-undecanone pyrolysate were evident in the liquid pyrolysis product of 2-tride~an0ne.l~Furthermore, the triolein pyrolysis product was also found to contain higher numbers of carbon atoms per double bond than predicted, suggesting the presence of C 1 product^.^ All these results indicated the existence of a third, significant pathway in the conversion of methyl ketones to hydrocarbons. For this pathway, it was proposed that methyl ketones were isomerized to aldehydes. These aldehydes then underwent oxidationheduction (disproportionation), either with themselves or with other ketones. For the reactions proposed here, the two aldehyde molecules produced a carboxylic acid (oxidized product) and a primary alcohol (reduced product). The alcohol then dehydrated to form monoalkenes, while the carboxylic acid again underwent addition with other acids in the system, producing ketones which were not necessarily symmetrical. These ketones could further undergo y-hydrogen transfer on one side to yield C-2 monoalkenes (nonenes in the case of 2-undecanone), and also on the other to give methyl ketones, which then reduced and dehydrated in the same manner as discussed before t o yield C 1 monoalkenes. For pyrolysis of the acetone/2-undecanone mixture (see above), the acetone was believed to reduce preferentially to 2-propanol, resulting in the preferential oxidation of the isomerized 2-undecanone. This increased C 1 monoalkene formation via production of a nonsymmetric C13 ketone. Dodecyl aldehyde pyrolysis appeared to support that some form of oxidationheduction occurred. The product's IR spectrum revealed C-H stretching bands typical for alkane/alkene mixtures in the 3000-2800 cm-l, as well as a complete absence of carbonyl. An unusually large C-H out-of-plane bending peak was also evident at 960 cm-l, as compared with the 905 cm-l alkene peak, which was much lower than in 2-undecanone pyrolysate spectra. Complete conversion to hydrocarbons was also indicated by carbon and hydrogen analysis (Table 2), where carbodhydrogen ratios were representative of monoalkenes. GC analysis also gave a uniform hydrocarbon mass distribution across the C6-Cl6 mass range, with multiple peaks for all mass fractions. Dominant hydrocarbons were those of C9 (10.5%), C10 (10.5%), and C12 (18.2%). Unlike methyl ketones, no C 1isomer doublet was observed, and significant hydrocarbon amounts were present for all carbon numbers. GC-MS further revealed each
mass fraction to consist of an alkane peak and several alkene isomers. Possible diene presence was also seen by peaks which were 2-4 mle units below the parent ions in alkene spectra. This suggested some superposition of diene spectra over those of alkenes. The formation of all critical mass fractions was explicable by the previously proposed disproportionation pathway. Hence, C10, C12, and C13 alkenes were formed by the oxidationheduction of two aldehyde molecules, while C9 alkenes were produced by y-hydrogen transfer on the aldehyde. C10, C11, and C14 alkenes were also predicted as disproportionation products between dodecyl aldehyde and the aldehyde formed from its corresponding methyl ketone, 2-tridecanone. Here, oxidation of the larger aldehyde and reduction of the smaller one should generally be preferred. The dominance of the C12 alkene fraction may have occurred from an additional, more direct reduction of the aldehyde. The C 1 Isomeric Ratio. The existence of only 1 monoalkenes in the pyrolysis product of two C 2-undecanone was intriguing, especially since a number of C9 and C11 monoalkene isomers were evident. A GC spectrum of this product spiked with l-dodecene con1 isomer was the clusively proved that neither C l-monoalkene, which was reasonable, as l-monoalkene formation was unfavorable thermodynamically. As a result, it was proposed that both compounds were actually the cis- and trans-2-alkene isomers. These were probably formed by methyl ketone reduction t o a 2-alcohol, which then dehydrated on the favorable side. The presence of both C 1isomers also appeared t o be unique to the pyrolysates of methyl ketones. Although neither carboxylic acids or triglycerides pyrolyzed to form notable quantities of these compounds, 2-undecanone and 2-tridecanone12pyrolysates exhibited 1 isomers in identical ratios. Pyrolysis of both C l-dodecanol also failed to yield this specific isomer distribution, instead producing five dodecene isomers. Furthermore, the ratio of the cis- and truns-2-alkenes varied little, ranging between 0.61 and 0.65 regardless of reactor retention time or extent of reaction. This was evident from comparison of GC product spectra for pyrolysate fractions corresponding to 40, 80, and 100 mL of 2-undecanone pyrolysis at a 0.42 h-l WHSV, as well as from 30 mL of 2-undecanone pyrolysis a t a lower 0.21 h-l WHSV. One reason for this specific ratio could be the manner in which substrate conversion over catalytic sites occurred. Studies of l-butene isomerization over activated alumina by Ghorbel et a1.,16Gerberich and Hall,17 and Maciver et a1.,18 all noted cis- and truns-2-butene to be the primary reaction products. The formation ratio of these isomers varied considerably with catalyst demoisturization temperature, but at 500-550 "C, both Maciver et al. and Gerberich and Hall reported ratios of 0.5-0.6. As the present studies employed catalyst dried at similar temperatures, it was believed that the catalytic surface, under these conditions, participated in a specific mechanism t o convert l-alkene t o its cisand trans-2-alkene isomers in a distinct ratio. It was unclear why only C 1monoalkenes produced this distribution. Therefore, it was proposed that double
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(16) Ghorbel, A., et. al. J . Catal. 1974, 33,123-132. (17)Gerberich, H. R.;Hall, W. K. J . Catal. 1966,5,99-110. (18)Maciver, D.S.,et. al. J . Catal. 1963, 2,485-497.
1096 Energy & Fuels, Vol. 9, No. 6, 1995
Vonghia et al.
bond and skeletal isomerization proceeded over different active sites. Consequently, for 2-undecanone pyrolysis, certain catalyst sites directly produced undecene isomers by skeletal isomerization, while dodecene formation proceeded via a l-alkene intermediate also formed from these sites, but later double-bond isomerized over other active sites.
way, such as the proposed disproportionation, need better verification and therefore merit further study. These results are significant in that they provide distinct pathways by which lipids deoxygenate to hydrocarbons over inorganic acidic catalysts such as activated alumina. The elucidation of these routes may also provide some insight into petroleum biogenesis, which may occur in a related manner.
Conclusions
Acknowledgment. The authors are grateful for the activated alumina catalyst provided by Pierre Chaulifour of Alcan Chemicals. Funding from the Natural Sciences and Engineering Research Council of Canada, in the form of operatinghtrategic grants and a scholarship (to E.V.), is gratefully acknowledged.
The pyrolysis of crude canola oil, 2-undecanone, and dodecyl aldehyde over activated alumina at WHSVs between 0.28 and 0.46 h-l, together with previous results, allowed for the proposal of an overall functional group pathway for the production of straight-chain alkenes from triglycerides. Some details of the path-
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